## Friday, August 22, 2014

### Small puzzle solution

Before I give my solution, I'd like to describe the leftmost digit algorithm in a bit more detail.
```(define (leftmost-digit base n)
(if (< n base)
n
(let ((leftmost-pair (leftmost-digit (* base base) n)))
(if (< leftmost-pair base)
leftmost-pair
(quotient leftmost-pair base)))))```
The idea is this: if we have a one digit number, we just return it, otherwise we recursively call `leftmost-digit` with the square of the base. Squaring the base will mean gobbling up pairs of digits in the recursive call, so we'll get back either a one or two digit answer from the recursion. If it is one digit, we return it, otherwise it's two digits and we divide by the base to get the left one.

For example, if our number is 12345678 and the base is 10, we'll make a recursive call with base 100. The recursive call will deal with number as if it were written `12 34 56 78` in base 100 and return the answer 12. Then we'll divide that by 10 to get the 1.

Since we're squaring the base, we're doubling the number of digits we're dealing with on each recursive call. This leads to the solution in O(log log n) time. If we instrument `quotient`, you can see:
```(leftmost-digit 10 816305093398751331727331379663195459013258742431006753294691576)
816305093398751331727331379663195459013258742431006753294691576 / 100000000000000000000000000000000
8163050933987513317273313796631 / 10000000000000000
816305093398751 / 100000000
8163050 / 10000
816 / 100```
A sixty-three digit number trimmed down to one digit with only five divisions.

So a simple solution to the puzzle is:
```(define (leftmost-digit+ base n)
(if (< n base)
(values n 0)
(call-with-values (lambda () (leftmost-digit+ (* base base) n))
(lambda (leftmost-pair count)
(if (< leftmost-pair base)
(values leftmost-pair (* count 2))
(values (quotient leftmost-pair base) (+ (* count 2) 1)))))))
```
The second value is the count of how many digits we discard. If the number is less than the base, we return it and we discarded nothing. Otherwise, we make the recursive call with the base squared and get back two values, the leftmost pair and the number of pairs it discarded. If the leftmost pair is a single digit, we return it, otherwise we divide by the base. The number of digits discarded is simply twice the number discarded by the recursive call, plus one more if we had to divide.

But I don't see an easy way to separate finding the digit from finding the position. At first it seemed straightforward to just count the digits being discarded, but you can't decide whether to increment the count at each stage without determining if the leftmost part of the recursive call contains one or two digits.

## Thursday, August 21, 2014

### Just a small puzzle

You can get the most significant digit (the leftmost) of a number pretty quickly this way
```(define (leftmost-digit base n)
(if (< n base)
n
(let ((leftmost-pair (leftmost-digit (* base base) n)))
(if (< leftmost-pair base)
leftmost-pair
(quotient leftmost-pair base)))))```
The puzzle is to adapt this code to return the position of the leftmost digit.

`(leftmost-digit+ 10 46729885)  would return two values, 4 and 7`

## Friday, August 8, 2014

### Mini regex golf 3: set cover

I'm computing the set cover by incrementally adding items to be covered. Naturally, the order in which you add items changes the way the program progresses. I added code that picks an item to be added each iteration rather than just pulling the `car` off the front of a list.
```(define (cover8 value->keys-table better-solution)

(let ((value (car v-k-entry))
(keys  (cdr v-k-entry)))

(write-string "Adding value ") (write value) (newline)
(write-string "   with keys ") (write keys) (newline)
(write-string "   to ") (write (length solution-set))
(write-string " partial solutions.") (newline)

(let ((new-solutions
(map make-new-solution (cartesian-product solution-set keys))))

(let ((trimmed-solutions
(trim-partial-solutions value->keys-table new-solutions)))

(write-string "Returning ") (write (length trimmed-solutions))
(write-string " of ") (write (length new-solutions))
(write-string " new partial solutions.") (newline)

trimmed-solutions))))

(define (cover v-k-entries)
(cond ((pair? v-k-entries)
(pick-v-k-entry value->keys-table v-k-entries
(lambda (selected remaining)
((null? v-k-entries)
(list '()))
(else (improper-list-error 'cover v-k-entries))))

(let ((minimized (minimize-vktable value->keys-table better-solution)))
(least-elements (cover minimized) better-solution)))

(define (score v-k-entry)
(let* ((matched-all
(count-matching-items value->keys-table
(lambda (other)
(there-exists? (cdr v-k-entry)
(lambda (key) (member key (cdr other)))))))
(matched-remaining
(count-matching-items v-k-entries
(lambda (other)
(there-exists? (cdr v-k-entry)
(lambda (key) (member key (cdr other)))))))
(matched-forward (- matched-all matched-remaining)))
(cons matched-remaining matched-forward)))

(let ((scored (map (lambda (v-k-entry) (cons (score v-k-entry) v-k-entry))
v-k-entries)))

(let ((picked
(cdar
(least-elements scored
(lambda (left right)
(let* ((len-l (length (cdr left)))
(len-r (length (cdr right)))
(lmr (caar left))
(lmf (cdar left))
(rmr (caar right))
(rmf (cdar right)))
(or (> len-l len-r)
(and (= len-l len-r)
(or (> lmf rmf)
(and (= lmf rmf)
(< lmr rmr)))))
))))))

(display "Picking ") (write picked) (newline)

(define (trim-partial-solutions value->keys-table partial-solutions)
(let ((equivalent-solutions
(map (lambda (entry) (cons (cdr entry) (car entry)))
(collect-equivalent-partial-solutions value->keys-table
partial-solutions))))
(write-string "  Deleting ")
(write (- (length partial-solutions) (length equivalent-solutions)))
(write-string " equivalent partial solutions.")
(newline)

(remove-dominated-solutions value->keys-table
(map lowest-scoring-equivalent-partial-solution
equivalent-solutions))))
```
Finally, it turns out that computing dominating partial solutions is expensive, so I changed the set operations to use a bitmap representation:
```(define (remove-dominated-solutions value->keys-table partial-solutions)
(let ((before-length (length partial-solutions))
(all-values (get-values value->keys-table)))
(let ((table
;; put the long ones in first
(sort
(map (lambda (partial-solution)
(cons partial-solution
(lset->bset all-values
(map car (partial-solution-matches value->keys-table
partial-solution)))))
partial-solutions)
(lambda (left right)
(> (length (bset->lset all-values (cdr left)))
(length (bset->lset all-values (cdr right))))))))

(dominates-solution? solution))
'()
table))))
(write-string "  Removing ") (write (- before-length after-length))
(write-string " dominated solutions.")
(newline)

(define (dominates-solution? solution)
(let* ((partial-solution (car solution))
(partial-solution-score (score partial-solution))
(solution-matches-raw (cdr solution)))
(lambda (other-solution)
(let* ((other-partial-solution (car other-solution))
(other-matches-raw (cdr other-solution)))
(and
(bset-superset? other-matches-raw solution-matches-raw)
(<= (score other-partial-solution) partial-solution-score))))))

(define (get-values v-k-table)
'()
v-k-table))

(define (bset-element->bit universe element)
(cond ((null? element) 0)
(else (expt 2 (list-index (lambda (item) (eq? item element)) universe)))))

(bset-union bset (bset-element->bit universe element)))

(define (lset->bset universe lset)
0
lset))

(define (bset->lset universe bset)
(cond ((zero? bset) '())
((even? bset) (bset->lset (cdr universe) (/ bset 2)))
(else (cons (car universe) (bset->lset (cdr universe) (/ (- bset 1) 2))))))

(define (bset-union left right) (bitwise-ior left right))

(define (bset-superset? bigger smaller)
;; Is every element of smaller in bigger?
(zero? (bitwise-andc2 smaller bigger)))
```
This code can now find the shortest regular expression consisting of letters and dots (and ^\$) that matches one set of strings but not another.

Depending on the strings, this can take quite a bit of time to run. Dotted expressions cause a combinatorical explosion in matching regexps (or substrings), but what makes it worse is that the dotted expressions tend to span different sets of strings. If two different dotted expressions, each with different matching sets of strings, appear in a single string, then the number of partial solutions will be multiplied by two as we try each different dotted expression.

It is characteristic of NP problems that it is easy to determine if you have a good solution, but quite hard to find it among a huge number of other, poor solutions. This problem exhibits this characteristic, but there is a bit more structure in the problem that we are exploiting. The word lists are drawn from the English language. This makes some bigrams, trigrams, etc. far, far, more likely to appear than others.

Short words are much easier to process than longer ones because they simply contain fewer things to match. On the other hand, longer words tend to be dominated by shorter ones anyway.

To be continued...

## Thursday, August 7, 2014

### Mini regex golf 2: adding regular expressions

It wasn't too hard to add regular expressions to the substring version. What took a while was just tinkering with the code, breaking it, fixing it again, noticing an optimization, tinkering, etc. etc. In any case it works and here is some of it.
```(define (make-extended-ngram-table winners losers)
(let* ((initial-ngrams (generate-ngrams winners losers)))
(write-string "Initial ngrams: ") (write (length initial-ngrams))
(newline)
(map (lambda (winner)
(cons winner
(keep-matching-items initial-ngrams
(lambda (ngram) (re-string-search-forward ngram winner)))))
winners)))

(define (generate-ngrams winners losers)
(write-string "Generating ngrams...")(newline)
(let ((losing-ngram? (string-list-matcher losers)))
(lset-union equal? answer (extended-ngrams losing-ngram? winner)))
'()
winners)))

(define (string-list-matcher string-list)
(lambda (test-ngram)
(there-exists? string-list
(lambda (string)
(re-string-search-forward test-ngram string)))))

(define *dotification-limit* 4)
(define *generate-ends-of-words* #t)
(define *generate-dotted* #t)

(define (ngrams-of-length n string)
(do ((start    0 (1+ start))
(end      n (1+ end))

(answer '() (let ((item (car tail)))
(if (losing-ngram? dotted)
(dotify item)))))
((not (pair? tail))
(if (null? tail)
(improper-list-error 'generate-dotted tail)))))

(define (dotify word)
(cond ((string=? word "") (list ""))
((> (string-length word) *dotification-limit*) (list word))
(else
(string-append replacement dotified)))
(replacements (substring word 0 1))))
'()
(dotify (substring word 1 (string-length word)))))))

(define (replacements string)
(if (or (string=? string "^")
(string=? string "\$"))
(list string)
(list string ".")))

(define (extended-ngrams losing-ngram? string)
(let ((string (if *generate-ends-of-words*
(string-append "^" string "\$")
string)))
(do ((n 1    (+ n 1))
(delete-matching-items (ngrams-of-length n string)
losing-ngram?))))
((> n (string-length string))
(if *generate-dotted*
Adding the dotification greatly increases the number of ways to match words:
```1 ]=> (extended-ngrams (string-list-matcher losers) "lincoln")

;Value 15: ("li" "ln" "ln\$" "oln" ".ln" "col" "lin" "li." "^li" "o.n\$" "oln\$" ".ln\$" "col." "c.ln" "..ln" "coln" ".oln" "co.n" "n.ol" "..ol" "ncol" ".col" "nc.l" "i.co" "inco" "i..o" "in.o" "lin." "li.." "l.nc" "linc" "l..c" "li.c" "^li." "^lin" "coln\$" "ncoln" "incol" "linco" "^linc" "ncoln\$" "incoln" "lincol" "^linco" "incoln\$" "lincoln" "^lincol" "lincoln\$" "^lincoln" "^lincoln\$")```
The table that maps words to their extended ngrams is quite large, but it can be reduced in size without affecting the solution to the set cover problem. If two regexps match exactly the same set of winning strings, then one can be substituted for the other in any solution, so we can discard all but the shortest of these. If a regexp matches a proper superset of another regexp, and the other regexp is at least the same length or longer, then the first regexp dominates the second one, so we can discard the second one.
```(define (minimize-keys value->keys-table better-solution)
(let* ((all-keys (get-keys value->keys-table))
(equivalents (collect-equivalent-partial-solutions value->keys-table
(map list all-keys)))
(reduced (map (lambda (equivalent)
(cons (car equivalent)
(car (least-elements (cdr equivalent)
better-solution))))
equivalents))
(dominants (collect-dominant-partial-solutions reduced better-solution))
'()
dominants)))

(define (rebuild-entry entry)
(cons (car entry) (keep-matching-items (cdr entry)
(lambda (item) (member item good-keys)))))

(write-string "Deleting ") (write (- (length all-keys) (length good-keys)))
(write-string " of ") (write (length all-keys)) (write-string " keys.  ")
(write (length good-keys)) (write-string " keys remain.")(newline)
(map rebuild-entry value->keys-table)))

(define (partial-solution-matches value->keys-table partial-solution)
(keep-matching-items
value->keys-table
(lambda (entry)
(there-exists? partial-solution (lambda (key) (member key (cdr entry)))))))

(define (collect-equivalent-partial-solutions value->keys-table partial-solutions)

(for-each (lambda (partial-solution)
(map car (partial-solution-matches
value->keys-table
partial-solution))
(list)
(lambda (other)
partial-solutions)

(define (collect-dominant-partial-solutions equivalents better-solution)
(define (dominates? left right)
(and (superset? (car left) (car right))
(not (better-solution (cdr right) (cdr left)))))

(let ((sorted (sort equivalents
(lambda (l r) (> (length (car l)) (length (car r)))))))
(if (there-exists? answer (lambda (a) (dominates? a candidate)))
'()
sorted)))
```
We can minimize the value->key-table in another way. If two values in the table are matched by the exact same set of keys, then we can delete one without changing the solution. If a value is matched by a small set of keys, and if another values is matched by a superset of these keys, then we can delete the larger one because if the smaller one matches, the larger one must match as well.
```(define (minimize-values v-k-table)
(let ((size-before (length v-k-table)))

(define (dominated-value? entry)
(let ((entry-value (car entry))
(entry-keylist (cdr entry)))
(there-exists? v-k-table
(lambda (other-entry)
(and (not (eq? entry other-entry))
(let ((other-value (car other-entry))
(other-keylist (cdr other-entry)))
(let ((result (and (superset? entry-keylist other-keylist)
(not (superset? other-keylist entry-keylist)))))
(if result
(begin (display "Removing ")
(write entry-value)
(display " dominated by ")
(write other-value)
(display ".")
(newline)
))
result)))))))

(let ((entry-value (car entry))
(entry-keylist (cdr entry)))
(lambda (other-entry)
(let ((other-value (car other-entry))
(other-keylist (cdr other-entry)))
(let ((result (equal? entry-keylist other-keylist)))
(if result
(begin (display "Removing ")
(write entry-value)
(display " equivalent to ")
(write other-value)
(display ".")
(newline)
))
result))))))

(dominated-value? entry))

(write-string "Removed ") (write (- size-before (length answer)))
(write-string " dominated and equivalent values.")
(newline)
Each time we remove values or keys, we might make more keys and values equivalent or dominated, so we iterate until we can no longer remove anything.
```(define (minimize-vktable value->keys-table better-solution)
(let* ((before-size (fold-left + 0 (map length value->keys-table)))
(new-table
(minimize-values
(minimize-keys value->keys-table better-solution)))
(after-size (fold-left + 0 (map length new-table))))
(if (= before-size after-size)
value->keys-table
(minimize-vktable new-table better-solution))))```
The minimized table for the presidents looks like this:
```(("washington" "sh" "g..n" "n..o" ".h.n" "a..i")
("monroe" "r.e\$" "oe")
("van-buren" "u..n" "r.n" ".b" "bu" "-")
("harrison" "r..s" "r.i" "i..n" "is." "i.o" "a..i")
("polk" "po")
("taylor" "ay." "ta")
("pierce" "ie." "rc" "r.e\$")
("buchanan" "bu" "a.a" ".h.n")
("lincoln" "i..o" "li")
("grant" "an.\$" "a.t" "ra" "r.n" "g..n")
("hayes" "h..e" "ye" "ay.")
("garfield" "el.\$" "i.l" "ga" "ie." "r.i" ".f" "a..i")
("cleveland" "v.l" "an.\$")
("mckinley" "n.e" "nl" "i.l" "m..i")
("roosevelt" ".se" "oo" "v.l" "el.\$" "r..s")
("taft" "a.t" "ta" ".f")
("wilson" "ls" "i..o")
("harding" "r.i" "di" "a..i")
("coolidge" "oo" "li")
("hoover" "ho" "oo")
("truman" "u..n" "ma")
("eisenhower" "ho" ".se" "h..e" "i..n" "is.")
("kennedy" "nn" "n.e")
("johnson" "j")
("nixon" "^n" "i..n" "i.o" "n..o")
("carter" "rt" "a.t")
("reagan" "ga" "a.a")
("bush" "bu" "sh")
("obama" ".b" "ma" "a.a" "am"))```
As you can see, we have reduced the original 2091 matching regexps to fifty.

Changes to the set-cover code coming soon....

## Friday, August 1, 2014

### Mini regex golf

I was intrigued by Peter Norvig's articles about regex golf.

To make things easier to think about, I decided to start with the simpler problem of looking for substrings. Here's code to extract the ngrams of a string:
```(define (ngrams-of-length n string)
(do ((start    0 (1+ start))
(end      n (1+ end))

(define (ngrams string)
(do ((n 1 (+ n 1))
A solution is simply a list of ngrams. (Although not every list of ngrams is a solution!)
```(define (solution? solution winners losers)
(let ((matches-solution? (ngram-list-matcher solution)))
(and (for-all? winners matches-solution?)
(not (there-exists? losers matches-solution?)))))

(define (ngram-list-matcher ngram-list)
(lambda (test-string)
(there-exists? ngram-list
(lambda (ngram)
(string-search-forward ngram test-string)))))
```
We also want to know if an ngram appears in a given list of strings.
```(define (string-list-matcher string-list)
(lambda (test-ngram)
(there-exists? string-list
(lambda (string)
(string-search-forward test-ngram string)))))

(let ((matches-loser? (string-list-matcher losers)))
(for-each
(lambda (winner) (write-string winner) (write-string ": ")
(write (reverse (delete-matching-items (ngrams winner) matches-loser?)))
(newline))
winners)))

washington: ("sh" "hi" "gt" "to" "was" "ash" "shi" "hin" "ngt" "gto" ...)
jefferson: ("j" "je" "ef" "ff" "fe" "rs" "jef" "eff" "ffe" "fer" ...)
monroe: ("oe" "onr" "nro" "roe" "monr" "onro" "nroe" "monro" "onroe" "monroe")
jackson: ("j" "ja" "ac" "ks" "jac" "ack" "cks" "kso" "jack" "acks" ...)
van-buren: ("-" "va" "n-" "-b" "bu" "van" "an-" "n-b" "-bu" "bur" ...)
harrison: ("har" "arr" "rri" "ris" "iso" "harr" "arri" "rris" "riso" "ison" ...)
polk: ("po" "pol" "olk" "polk")
taylor: ("ta" "yl" "lo" "tay" "ayl" "ylo" "lor" "tayl" "aylo" "ylor" ...)
pierce: ("rc" "ce" "pie" "ier" "erc" "rce" "pier" "ierc" "erce" "pierc" ...)
buchanan: ("bu" "uc" "ch" "na" "buc" "uch" "cha" "ana" "nan" "buch" ...)
lincoln: ("li" "ln" "lin" "col" "oln" "linc" "inco" "ncol" "coln" "linco" ...)
grant: ("ra" "gra" "ran" "ant" "gran" "rant" "grant")
hayes: ("ye" "hay" "aye" "yes" "haye" "ayes" "hayes")
garfield: ("ga" "rf" "fi" "gar" "arf" "rfi" "fie" "iel" "eld" "garf" ...)
cleveland: ("lev" "vel" "ela" "clev" "leve" "evel" "vela" "elan" "cleve" "level" ...)
mckinley: ("nl" "mck" "inl" "nle" "mcki" "kinl" "inle" "nley" "mckin" "ckinl" ...)
roosevelt: ("oo" "os" "lt" "roo" "oos" "ose" "sev" "vel" "elt" "roos" ...)
taft: ("ta" "af" "ft" "taf" "aft" "taft")
wilson: ("ls" "ils" "lso" "wils" "ilso" "lson" "wilso" "ilson" "wilson")
harding: ("di" "har" "ard" "rdi" "din" "hard" "ardi" "rdin" "ding" "hardi" ...)
coolidge: ("oo" "li" "coo" "ool" "oli" "lid" "cool" "ooli" "olid" "lidg" ...)
hoover: ("ho" "oo" "hoo" "oov" "hoov" "oove" "hoove" "oover" "hoover")
truman: ("tr" "ru" "ma" "tru" "rum" "uma" "man" "trum" "ruma" "uman" ...)
eisenhower: ("ei" "nh" "ho" "ow" "eis" "ise" "sen" "enh" "nho" "how" ...)
kennedy: ("nn" "ed" "dy" "ken" "enn" "nne" "ned" "edy" "kenn" "enne" ...)
johnson: ("j" "jo" "oh" "hn" "joh" "ohn" "hns" "john" "ohns" "hnso" ...)
nixon: ("ni" "ix" "xo" "nix" "ixo" "xon" "nixo" "ixon" "nixon")
carter: ("rt" "car" "art" "rte" "cart" "arte" "rter" "carte" "arter" "carter")
reagan: ("ea" "ag" "ga" "rea" "eag" "aga" "gan" "reag" "eaga" "agan" ...)
bush: ("bu" "us" "sh" "bus" "ush" "bush")
clinton: ("li" "to" "cli" "lin" "int" "nto" "ton" "clin" "lint" "into" ...)
obama: ("ob" "ba" "am" "ma" "oba" "bam" "ama" "obam" "bama" "obama")
```
We can discard ngrams like "shi" because the shorter ngram "sh" will also match.
```(define (dominant-ngrams string losing-ngram?)
(do ((n 1 (+ n 1))
(delete-matching-items
(ngrams-of-length n string)
(lambda (item)
(lambda (ngram)
(string-search-forward ngram item)))
(losing-ngram? item))))

(let ((matches-loser? (string-list-matcher losers)))
(for-each
(lambda (winner) (write-string winner) (write-string ": ")
(write (dominant-ngrams winner matches-loser?))
(newline))
winners)))

washington: ("was" "to" "gt" "hi" "sh")
jefferson: ("rs" "fe" "ff" "ef" "j")
monroe: ("nro" "onr" "oe")
jackson: ("ks" "ac" "j")
van-buren: ("ren" "ure" "bu" "va" "-")
harrison: ("iso" "ris" "rri" "arr" "har")
polk: ("olk" "po")
taylor: ("lo" "yl" "ta")
pierce: ("ier" "pie" "ce" "rc")
buchanan: ("na" "ch" "uc" "bu")
lincoln: ("inco" "col" "ln" "li")
grant: ("ant" "ra")
hayes: ("hay" "ye")
garfield: ("eld" "iel" "fi" "rf" "ga")
cleveland: ("ela" "vel" "lev")
mckinley: ("mck" "nl")
roosevelt: ("vel" "sev" "lt" "os" "oo")
taft: ("ft" "af" "ta")
wilson: ("ls")
harding: ("ard" "har" "di")
coolidge: ("li" "oo")
hoover: ("oo" "ho")
truman: ("ma" "ru" "tr")
eisenhower: ("wer" "sen" "ise" "ow" "ho" "nh" "ei")
kennedy: ("ken" "dy" "ed" "nn")
johnson: ("hn" "oh" "j")
nixon: ("xo" "ix" "ni")
carter: ("car" "rt")
reagan: ("ga" "ag" "ea")
bush: ("sh" "us" "bu")
clinton: ("int" "to" "li")
obama: ("ma" "am" "ba" "ob")
```
It's time to tackle the set cover problem. We want a set of ngrams that match all the strings. Obviously, if we pick an ngram from each of the strings we want to cover, we'll have a solution. For instance,
```(let ((matches-loser? (string-list-matcher losers)))
(solution? (delete-duplicates
(map
(lambda (winner) (car (dominant-ngrams winner matches-loser?)))
winners))
winners losers))
;Value: #t
```
We can cycle through all the possible solutions and then select the best one.
```(define (mini-golf0 winners losers)
(lowest-scoring
(cover0 (make-dominant-ngram-table
winners
(delete-losing-superstrings winners losers)))))

(define (delete-losing-superstrings winners losers)
(delete-matching-items
losers
(lambda (loser)
(there-exists? winners
(lambda (winner)
(string-search-forward winner loser))))))

(define (make-dominant-ngram-table winners losers)
(let ((losing-ngram? (string-list-matcher losers)))
(map (lambda (winner)
(cons winner (dominant-ngrams winner losing-ngram?)))
winners)))

(define (cover0 v-k-table)
(let ((empty-solution-set (list '())))

(let ((value (car v-k-entry))
(keys  (cdr v-k-entry)))

(write-string "Adding value ") (write value) (newline)
(write-string "   with keys ") (write keys) (newline)
(write-string "   to ") (write (length solution-set))
(write-string " partial solutions.") (newline)

(let ((new-solutions
(map make-new-solution (cartesian-product solution-set keys))))

(write-string "Returning ") (write (length new-solutions))
(write-string " new partial solutions.") (newline)

new-solutions)))

(define (lowest-scoring list)
(least-elements list (lambda (l r) (< (score l) (score r)))))

(define (cartesian-product left-list right-list)
right-list))
'()
left-list))

(define (make-new-solution cp-term)
(let ((solution (car cp-term))
(key (cdr cp-term)))

(define (improper-list-error procedure thing)
(error (string-append "Improper list found by " procedure ": ") thing))

(define (least-elements list <)
((< item (car answer)) (cons item '()))

(cond ((pair? list) (fold-left accumulate-least
(cons (car list) '())
(cdr list)))
((null? list) (error "List must have at least one element." list))
(else (improper-list-error 'LEAST-ELEMENTS list))))

(define (score solution)
(do ((tail solution (cdr tail))
(score -1      (+ score (string-length (car tail)) 1)))
((not (pair? tail))
(if (null? tail)
score
(improper-list-error 'score solution)))))
```
This works for small sets:
```1 ]=> (mini-golf0 boys girls)
with keys ("ob" "c" "j")
to 1 partial solutions.
Returning 3 new partial solutions.
with keys ("as")
to 3 partial solutions.
Returning 3 new partial solutions.
with keys ("an" "ha")
to 3 partial solutions.
Returning 6 new partial solutions.
with keys ("ah" "oa" "no")
to 6 partial solutions.
Returning 18 new partial solutions.
with keys ("lia" "lli" "ill" "am" "w")
to 18 partial solutions.
Returning 90 new partial solutions.
with keys ("lia" "am")
to 90 partial solutions.
Returning 180 new partial solutions.
with keys ("en" "de" "yd" "ay" "j")
to 180 partial solutions.
Returning 900 new partial solutions.
with keys ("ae" "ha" "c")
to 900 partial solutions.
Returning 2700 new partial solutions.
with keys ("de" "nd" "an" "le" "al" "r" "x")
to 2700 partial solutions.
Returning 18900 new partial solutions.
with keys ("en" "de" "id")
to 18900 partial solutions.
Returning 56700 new partial solutions.
;Value 41: (("de" "am" "ah" "ha" "as" "j")
("de" "am" "ah" "ha" "as" "j")
("de" "am" "oa" "ha" "as" "j")
("de" "am" "oa" "ha" "as" "j")
("de" "am" "no" "ha" "as" "j")
("de" "am" "no" "ha" "as" "j")
("de" "am" "ah" "ha" "as" "c")
("de" "am" "ah" "ha" "as" "c")
("de" "am" "oa" "ha" "as" "c")
("de" "am" "oa" "ha" "as" "c")
("de" "am" "no" "ha" "as" "c")
("de" "am" "no" "ha" "as" "c")
("de" "am" "ah" "an" "as" "c")
("de" "am" "ah" "an" "as" "c")
("en" "am" "ah" "an" "as" "c")
("de" "am" "oa" "an" "as" "c")
("de" "am" "oa" "an" "as" "c")
("en" "am" "oa" "an" "as" "c")
("de" "am" "no" "an" "as" "c")
("de" "am" "no" "an" "as" "c")
("en" "am" "no" "an" "as" "c"))
```
But you can see that we won't be able to go much beyond this because there are just too many combinations. We can cut down on the intermediate partial solutions by noticing that many of them are redundant. We don't need to keep partial solutions that cannot possibly lead to a shortest final solution. The various partial solutions each (potentially) match different sets of words. We only need keep the shortest solution for each different set of matched words. Furthermore, if a solution's matches are a superset of another's matches, and the other is the same length or longer, then the solution is dominated by the other and will always be at least the length of the longer.
```(define (mini-golf1 winners losers)
(cover1
(make-dominant-ngram-table winners (delete-losing-superstrings winners losers))
lowest-scoring))

(define (cover1 v-k-table lowest-scoring)
(let ((empty-solution-set (list '())))

(let ((value (car v-k-entry))
(keys  (cdr v-k-entry)))

(write-string "Adding value ") (write value) (newline)
(write-string "   with keys ") (write keys) (newline)
(write-string "   to ") (write (length solution-set))
(write-string " partial solutions.") (newline)

(let ((new-solutions
(map make-new-solution (cartesian-product solution-set keys))))

(let ((trimmed-solutions (trim-partial-solutions new-solutions)))

(write-string "Returning ") (write (length trimmed-solutions))
(write-string " of ") (write (length new-solutions))
(write-string " new partial solutions.") (newline)

trimmed-solutions))))

(define (trim-partial-solutions partial-solutions)
(let ((equivalent-solutions (collect-equivalent-partial-solutions partial-solutions)))
(write-string "  Deleting ")
(write (- (length partial-solutions) (length equivalent-solutions)))
(write-string " equivalent partial solutions.")
(newline)

(remove-dominated-solutions
(map lowest-scoring-equivalent-partial-solution equivalent-solutions))))

(define (lowest-scoring-equivalent-partial-solution entry)
(first (lowest-scoring (car entry))))

(define (collect-equivalent-partial-solutions alist)
;; Add each entry in turn.
(fold-left (lambda (equivalents partial-solution)
partial-solution
(partial-solution-matches partial-solution)
equivalents))
'() alist))

(define (partial-solution-matches partial-solution)
(keep-matching-items v-k-table
(lambda (entry)
(there-exists? partial-solution
(lambda (key) (member key (cdr entry)))))))

(define (remove-dominated-solutions partial-solutions)
(let ((before-length (length partial-solutions)))
'()
(map (lambda (partial-solution)
(cons partial-solution (partial-solution-matches partial-solution)))
partial-solutions)))))
(write-string "  Deleting ") (write (- before-length after-length))
(write-string " dominated solutions.")
(newline)

(lowest-scoring

(define (dominates-solution? solution)
(let ((partial-solution (car solution))
(solution-matches (cdr solution)))
(lambda (other-solution)
(let ((other-partial-solution (car other-solution))
(other-matches (cdr other-solution)))
(and (not (equal? solution-matches other-matches))
(superset? other-matches solution-matches)
(<= (score other-partial-solution) (score partial-solution)))))))

(cond ((pair? alist)
(let ((entry (car alist))
(tail (cdr alist)))
(let ((entry-solutions (car entry))
(entry-value (cdr entry)))
(if (equal? value entry-value)
(if (member solution entry-solutions)
alist
(cons (cons (cons solution entry-solutions) value)
tail))
(cons entry (add-equivalent-partial-solution solution value tail))))))
((null? alist) (list (cons (list solution) value)))
(else (improper-list-error 'collect-equivalents alist))))
```
```1 ]=> (mini-golf1 winners losers)
with keys ("was" "to" "gt" "hi" "sh")
to 1 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 1 dominated solutions.
Returning 2 of 5 new partial solutions.
to 2 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 2 dominated solutions.
Returning 4 of 6 new partial solutions.
with keys ("rs" "fe" "ff" "ef" "j")
to 4 partial solutions.
Deleting 12 equivalent partial solutions.
Removing 4 dominated solutions.
Returning 4 of 20 new partial solutions.
with keys ("iso" "di" "ad" "ma")
to 4 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 2 dominated solutions.
Returning 12 of 16 new partial solutions.
with keys ("nro" "onr" "oe")
to 12 partial solutions.
Deleting 24 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 12 of 36 new partial solutions.
with keys ("ks" "ac" "j")
to 12 partial solutions.
Deleting 24 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 12 of 36 new partial solutions.
with keys ("ren" "ure" "bu" "va" "-")
to 12 partial solutions.
Deleting 36 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 24 of 60 new partial solutions.
with keys ("iso" "ris" "rri" "arr" "har")
to 24 partial solutions.
Deleting 96 equivalent partial solutions.
Removing 12 dominated solutions.
Returning 12 of 120 new partial solutions.
with keys ("olk" "po")
to 12 partial solutions.
Deleting 12 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 12 of 24 new partial solutions.
with keys ("lo" "yl" "ta")
to 12 partial solutions.
Deleting 12 equivalent partial solutions.
Removing 12 dominated solutions.
Returning 12 of 36 new partial solutions.
with keys ("ier" "pie" "ce" "rc")
to 12 partial solutions.
Deleting 36 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 12 of 48 new partial solutions.
with keys ("na" "ch" "uc" "bu")
to 12 partial solutions.
Deleting 39 equivalent partial solutions.
Removing 3 dominated solutions.
Returning 6 of 48 new partial solutions.
with keys ("inco" "col" "ln" "li")
to 6 partial solutions.
Deleting 15 equivalent partial solutions.
Removing 6 dominated solutions.
Returning 3 of 24 new partial solutions.
with keys ("ant" "ra")
to 3 partial solutions.
Deleting 3 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 3 of 6 new partial solutions.
with keys ("hay" "ye")
to 3 partial solutions.
Deleting 3 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 3 of 6 new partial solutions.
with keys ("eld" "iel" "fi" "rf" "ga")
to 3 partial solutions.
Deleting 9 equivalent partial solutions.
Removing 3 dominated solutions.
Returning 3 of 15 new partial solutions.
with keys ("ela" "vel" "lev")
to 3 partial solutions.
Deleting 3 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 9 new partial solutions.
with keys ("mck" "nl")
to 6 partial solutions.
Deleting 6 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 12 new partial solutions.
with keys ("vel" "sev" "lt" "os" "oo")
to 6 partial solutions.
Deleting 24 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 30 new partial solutions.
with keys ("ft" "af" "ta")
to 6 partial solutions.
Deleting 12 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 18 new partial solutions.
with keys ("ls")
to 6 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 6 new partial solutions.
with keys ("ard" "har" "di")
to 6 partial solutions.
Deleting 12 equivalent partial solutions.
Removing 2 dominated solutions.
Returning 4 of 18 new partial solutions.
with keys ("li" "oo")
to 4 partial solutions.
Deleting 4 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 4 of 8 new partial solutions.
with keys ("oo" "ho")
to 4 partial solutions.
Deleting 4 equivalent partial solutions.
Removing 2 dominated solutions.
Returning 2 of 8 new partial solutions.
with keys ("ma" "ru" "tr")
to 2 partial solutions.
Deleting 4 equivalent partial solutions.
Removing 1 dominated solutions.
Returning 1 of 6 new partial solutions.
with keys ("wer" "sen" "ise" "ow" "ho" "nh" "ei")
to 1 partial solutions.
Deleting 6 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 7 new partial solutions.
with keys ("ken" "dy" "ed" "nn")
to 1 partial solutions.
Deleting 3 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 4 new partial solutions.
with keys ("hn" "oh" "j")
to 1 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 3 new partial solutions.
with keys ("xo" "ix" "ni")
to 1 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 3 new partial solutions.
with keys ("car" "rt")
to 1 partial solutions.
Deleting 1 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 2 new partial solutions.
with keys ("ga" "ag" "ea")
to 1 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 3 new partial solutions.
with keys ("sh" "us" "bu")
to 1 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 3 new partial solutions.
with keys ("int" "to" "li")
to 1 partial solutions.
Deleting 2 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 3 new partial solutions.
with keys ("ma" "am" "ba" "ob")
to 1 partial solutions.
Deleting 3 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 4 new partial solutions.
;Value 47: (("rt" "ni" "nn" "ho" "ls" "nl" "vel" "ga" "ye" "ra" "li" "rc" "ta" "po" "har" "bu" "oe" "ma" "j" "ad" "sh"))```
The `cover` procedure takes a table that maps values to the keys that cover them. If we can reduce the size of that table without changing the solution, we'll run faster. If there are two entries in the table such that the keys of one are a superset of the keys of the other, we can discard the superset: the smaller of the two entries will be in the solution, and any key that matches the smaller one will automatically match the larger one as well. Also, if two values have the same set of keys that match them, we need only include one of the values in the table.
```(define (delete-dominated-values v-k-table)
(let ((size-before (length v-k-table)))

(define (dominated-value? entry)
(let ((entry-value (car entry))
(entry-keylist (cdr entry)))
(there-exists? v-k-table
(lambda (other-entry)
(and (not (eq? entry other-entry))
(let ((other-value (car other-entry))
(other-keylist (cdr other-entry)))
(and (superset? entry-keylist other-keylist)
(not (equal? other-keylist entry-keylist)))))))))

(let ((entry-value (car entry))
(entry-keylist (cdr entry)))
(lambda (other-entry)
(let ((other-value (car other-entry))
(other-keylist (cdr other-entry)))
(equal? entry-keylist other-keylist))))))

(dominated-value? entry))

(write-string "Removed ") (write (- size-before (length answer)))
(write-string " dominated and equivalent values.")
(newline)

(define (superset? bigger smaller)
(for-all? smaller (lambda (s) (member s bigger))))

(define (mini-golf2 winners losers)
(cover1
(delete-dominated-values
(make-dominant-ngram-table winners (delete-losing-superstrings winners losers)))
lowest-scoring))

;;;;;;;;
;; Delete dominated keys from the keylists.

(define (mini-golf3 winners losers)
(cover1
(delete-dominated-keys-and-values
(make-dominant-ngram-table winners (delete-losing-superstrings winners losers))
(lambda (left right)
(or (< (string-length left) (string-length right))
(and (= (string-length left) (string-length right))
(string<? left right)))))
lowest-scoring))

(define (delete-dominated-keys-and-values v-k-table better-key)
(let ((before-size (fold-left * 1 (map length v-k-table))))
(let ((new-table (delete-dominated-values
(delete-dominated-keys v-k-table better-key))))
(let ((after-size (fold-left * 1 (map length new-table))))
(if (= before-size after-size)
v-k-table
(delete-dominated-keys-and-values new-table better-key))))))

(define (delete-dominated-keys v-k-table better-key)
(let ((all-keys (get-all-keys v-k-table)))

(define (lookup-key key)
(cons key
(map car
(keep-matching-items v-k-table
(lambda (v-k-entry)
(member key (cdr v-k-entry)))))))

(let ((k-v-table (map lookup-key all-keys)))

(define (dominated-key? key)
(let ((values (cdr (assoc key k-v-table))))
(there-exists? k-v-table
(lambda (entry)
(let ((entry-key (car entry))
(entry-values (cdr entry)))
(and (superset? entry-values values)
(not (equal? values entry-values))
(or (< (string-length entry-key) (string-length key))
(and (= (string-length entry-key) (string-length key))
(string<? entry-key key)))))))))

(let ((values (cdr (assoc key k-v-table))))
(lambda (entry-key)
(let ((entry-values (cdr (lookup-key entry-key))))
(equal? values entry-values))))))

(if (or (dominated-key? key)

(let ((good-keys (fold-left add-keys '() (sort all-keys better-key))))
(write-string "Removed ") (write (- (length all-keys) (length good-keys)))
(write-string " of ") (write (length all-keys)) (write-string " keys.")(newline)

(map (lambda (entry)
(cons (car entry)
(keep-matching-items (cdr entry) (lambda (key) (member key good-keys)))))
v-k-table)))))

(define (get-all-keys v-k-table)
(cdr entry)))
'()
v-k-table))```
Trimming the table this way helps a lot. We can now compute the dogs vs. cats.
```1 ]=> (mini-golf3 dogs cats)

Removed 294 of 405 keys.
Removed 44 dominated and equivalent values.
Removed 25 of 93 keys.
Removed 15 dominated and equivalent values.
Removed 7 of 62 keys.
Removed 0 dominated and equivalent values.
Removed 0 of 55 keys.
Removed 0 dominated and equivalent values.
with keys ("OIS" "BOR" "RZ")
to 1 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 3 of 3 new partial solutions.
with keys ("SCH" "HN")
to 3 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 6 new partial solutions.
with keys ("JI")
to 6 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 6 of 6 new partial solutions.
with keys ("TERS" "ETT")
to 6 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 12 of 12 new partial solutions.
with keys ("CHI")
to 12 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 12 of 12 new partial solutions.
with keys ("S F" "DES" " DE" "IER" "FL" "VI")
to 12 partial solutions.
Deleting 8 equivalent partial solutions.
Removing 8 dominated solutions.
Returning 56 of 72 new partial solutions.
with keys ("EKI")
to 56 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 56 of 56 new partial solutions.
with keys (" MAL" "OIS" "LG")
to 56 partial solutions.
Deleting 96 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 72 of 168 new partial solutions.
with keys ("TERS" "D P")
to 72 partial solutions.
Deleting 108 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 36 of 144 new partial solutions.
with keys ("W ")
to 36 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 36 of 36 new partial solutions.
with keys ("DS")
to 36 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 36 of 36 new partial solutions.
with keys ("BOR" " DE" "GU")
to 36 partial solutions.
Deleting 88 equivalent partial solutions.
Removing 2 dominated solutions.
Returning 18 of 108 new partial solutions.
with keys ("ANS" "LM")
to 18 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 36 of 36 new partial solutions.
with keys ("LH")
to 36 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 36 of 36 new partial solutions.
with keys (" COR" "ORS")
to 36 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 72 of 72 new partial solutions.
with keys (" MAL" "TES" "LAS" "KA")
to 72 partial solutions.
Deleting 184 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 104 of 288 new partial solutions.
with keys ("IP")
to 104 partial solutions.
Deleting 0 equivalent partial solutions.
;GC #199: took:   0.20   (1%) CPU time,   0.10   (1%) real time; free: 16754359
Removing 0 dominated solutions.
Returning 104 of 104 new partial solutions.
with keys ("SHI" " I")
to 104 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 208 of 208 new partial solutions.
with keys ("AK")
to 208 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 208 of 208 new partial solutions.
with keys ("DES" "DG" "OD")
to 208 partial solutions.
Deleting 304 equivalent partial solutions.
Removing 144 dominated solutions.
Returning 176 of 624 new partial solutions.
with keys ("S F" "FR")
to 176 partial solutions.
Deleting 224 equivalent partial solutions.
Removing 16 dominated solutions.
Returning 112 of 352 new partial solutions.
with keys ("API")
to 112 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 112 of 112 new partial solutions.
with keys ("IES")
to 112 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 112 of 112 new partial solutions.
with keys ("LAS" "IZ" "VI")
to 112 partial solutions.
;GC #200: took:   0.10   (0%) CPU time,   0.10   (1%) real time; free: 16757322
Deleting 272 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 64 of 336 new partial solutions.
with keys ("ITT")
to 64 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 64 of 64 new partial solutions.
with keys ("GS")
to 64 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 64 of 64 new partial solutions.
with keys ("HAVANE")
to 64 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 64 of 64 new partial solutions.
with keys ("ANI" "LS")
to 64 partial solutions.
Deleting 80 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 48 of 128 new partial solutions.
with keys ("FS")
to 48 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 48 of 48 new partial solutions.
with keys ("TES" "LT")
to 48 partial solutions.
Deleting 72 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 24 of 96 new partial solutions.
with keys (" COR" "LS")
to 24 partial solutions.
Deleting 32 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 16 of 48 new partial solutions.
with keys ("IER" " T")
to 16 partial solutions.
Deleting 24 equivalent partial solutions.
Removing 4 dominated solutions.
Returning 4 of 32 new partial solutions.
with keys ("ANS" "ANI")
to 4 partial solutions.
Deleting 6 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 2 of 8 new partial solutions.
with keys ("GR")
to 2 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 2 of 2 new partial solutions.
with keys ("SCH" " PI")
to 2 partial solutions.
Deleting 3 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 4 new partial solutions.
with keys ("SHI" " T")
to 1 partial solutions.
Deleting 1 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 2 new partial solutions.
with keys ("EI")
to 1 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 1 new partial solutions.
with keys ("DL" "OD")
to 1 partial solutions.
Deleting 1 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 2 new partial solutions.
with keys ("OX")
to 1 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 1 new partial solutions.
with keys ("AGL")
to 1 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 1 new partial solutions.
with keys ("VE")
to 1 partial solutions.
Deleting 0 equivalent partial solutions.
Removing 0 dominated solutions.
Returning 1 of 1 new partial solutions.
;Value 50: (("VE" "AGL" "OX" "EI" "GR" " T" "FS" "LS" "HAVANE" "GS" "ITT" "IES" "API" "FR" "OD" "AK" " I" "IP" "TES" "ORS" "LH" "ANS" "GU" "DS" "W " "EKI" "VI" "CHI" "TERS" "JI" "SCH" "OIS"))```
We appear to have the substring version of regex golf under control. Can we extend it to actual regular expressions? Of course we can. In the next installment...

## Friday, October 18, 2013

### Unexciting code

Source code control isn't supposed to be interesting or exciting for the user. Blogging about source code control isn't going to be interesting or surprising for the reader, either. I'll spare you the walkthrough. You're old enough to figure it out without me holding your hand. It'll be more interesting if I blog about the problems and bugs we encountered.

Conman was the configuration manager built on the core ChangeSafe code. The master catalog in conman holds the highest level objects being managed.
```(defclass master-catalog ()
;; subsystems and products (PC's) in this repository
((products   :initform nil
:version-technique :composite-set
:accessor master-catalog/products)

(subsystems :initform nil
:version-technique :composite-set
:accessor master-catalog/subsystems)

;; All satellite repositories known to this master.  We don't allow
;; satellite repository names to change, though the projects they
;; contain may be renamed.  **WARNING** BE CAREFUL IF YOU OPERATE
;; in non-latest metaversion views of this list or you might try to
;; create a satellite which already exists.  Only update this list
;; using the latest metaversion.
(satellite-repositories :initform nil :version-technique :composite-set)

;; Projects (a.k.a. ChangeSafe "classes") contained in satellite
;; repositories.  The descriptors contain the key mapping from a
;; project name to a satellite-repository-name.  We could almost
;; make this just a (project-name . satellite-name) mapping, but we
;; need to version project-name, and we also want to cache in the
;; master some information in the satellites so that we don't
;; always have to examine the satellites for often accessed
;; information.
(classes :accessor master-catalog/classes
:initform nil
:version-technique :composite-set)

;; Cset-relationship-tuples is conceptually a list of sublists,
;; where each sublist is a tuple.  For every master cid which
;; results in the creation of satellite cids, a tuple is added
;; which enumerates the master cid and the satellite cids which it
;; caused to be created.  e.g. '((master.cid.1 satellite-1.cid.1
;; satellite-2.cid.1)) Because we want portable references, blah
;; blah blah, we actually reference DIDS of CHANGE-SET objects
;; rather than the cids.  We may instead wish to store CID-OBJECT
;; references.  TBD.

;; Right now, this information is maintained only for change
;; transactions which arise from WITH-CM-MASTER-TXN and
;; WITH-CM-SATELLITE-TXN.  This is ok, since those are the
;; interesting txns which manipulate satellite databases.

;; Note that because of the high volume of csets we expect to
;; track, we actually represent this information as a vector of
;; vectors to achieve space compaction.
(cset-relationship-tuples :initform (make-instance 'persistent-vector
:initial-element nil
:size 1)
:version-technique :nonversioned)

(cset-rel-tuples-index :initform (make-instance 'persistent-vector
:initial-element -1
:size 1)
:version-technique :nonversioned)

;; BOTH these slots are updated ONLY by vm-txn-note-change-set,

;; The cset-rel-tuples-index slot is a conceptual hash table into the
;; cset-relationship-tuples slot. This is used by
;; master-catalog-lookup-cset-relationship
;; to avoid an extremely costly linear search of cset-relationship-tuples.
;; This is very important for cset_add, cset_remove, and csets_from_file.

;; The idea is that the did-string of the master-cid's did is hashed.
;; Reducing that hash modulo the number of entries in cset-rel-tuples-index,
;; finds a "home" index of cset-rel-tuples-index. Using the sb32 value
;; in that element, we either have a -1 (so the entry is not in the
;; hash table) or we get an index into cset_relationship_tuples.
;; If there is no hash collision, that element of cset_relationship_tuples
;; will contain the desired master-cid did we are looking for. If it
;; isn't the one we want, we have had a hash collision, and we resolve it
;; by linear probing in the next-door (circularly) element of
;; cset-rel-tuples-index.
;; The number of elements of cset-rel-tuples-index is always a prime number,
;; and is also maintained to be more than twice as large as the number of
;; entries in cset-relationship-tuples. That is important, to prevent
;; clustering and slow searching. So when it grows, cset-rel-tuples-index
;; grows by a reasonable factor (about 2) so that it always contains
;; at least half "holes", that is, -1.  Further, we want to avoid frequent
;; growth, because growing requires computing every entry in the hash table
;; again. That makes for a big transaction, as every element of the
;; cid-relationship-tuple vector has to be mapped in, and rehashed with
;; the new size of cset-rel-tuples-index.
;; Space considerations: In Jack's db, there are roughly 40,000 elements
;; currently in the cset-relationship-tuples.  Suppose we had 100,000
;; elements. In Jack's db, it appears that the tuples are about 2 elements
;; each, average. Suppose it were 9. Then the tuples would take 4*(1+9)=40
;; bytes each, so 40*100,000 = 4Mb total (plus another 400,000 for the
;; cset-relationship-tuples vector itself).  This is large, but not likely
;; to be a cause of breakage anytime soon.

;; The cset-name-hashtable maps cset names to csets.  While the HP
;; model of ChangeSafe doesn't allow changing the name of a cset,
;; we allow this in general.  So this hash table is keyed by cset
;; name, and valued by all csets which EVER bore that name in their
;; versioned name component.  The hash value is therefore a list.
;; In the case of system augmented names (by
;; change_create/master_change), there shouldn't be any collisions.
;; We also use this slot to hash unaugmented user names to csets,
;; and those are far more likely to have collisions (one key ->
;; multiple csets).  In the case of un-augmented names, this is
;; expected. In the case of augmented names, this is an error.
(cset-name-hashtable :version-technique nil
:initform (make-instance 'persistent-hash-table :size 1023)
)
(:documentation "Catalog/hierarchy-root of versioned information maintained in the master repository.")
(:metaclass versioned-standard-class)
(:schema-version 0))```
Pretty straightforward, no? No? Let's list the products in the master catalog:
```(defun cmctl/list-products (conman-request)
"cheesy hack to list the products"
(let ((master-repository-name (conman-request/repository-dbpath conman-request))
(reason "list products")
(userid (conman-request/user-name conman-request)))
(call-with-master-catalog-transaction
master-repository-name
userid
:master-metaversion :latest-metaversion
:version :latest-version
:reason reason
(declare (ignore master-repository master-transaction))
(collect 'list
(map-fn 't (lambda (product)
(list (distributed-object-identifier product)
(named-object/name product)
(described-object/description product)))
(master-catalog/scan-products master-catalog)))))))```
That isn't very edifying.

Start from the end:
`(master-catalog/scan-products master-catalog)`
defined as
```(defun master-catalog/scan-products (master-catalog)
(declare (optimizable-series-function))
(versioned-object/scan-composite-versioned-slot master-catalog 'products))```
The `optimizable-series-function` declaration indicates that we are using Richard Waters's series package. This allows us to write functions that can be assembled into an efficient pipeline for iterating over a collection of objects. This code:
```(collect 'list
(map-fn 't (lambda (product)
(list (distributed-object-identifier product)
(named-object/name product)
(described-object/description product)))
(master-catalog/scan-products master-catalog)))```
takes each product in turn, creates a three element list of the identifier, the project name, and the product description, and finally collects the three-tuples in a list to be returned to the caller. Here is what it looks like macroexpanded:
```(COMMON-LISP:LET* ((#:OUT-917 MASTER-CATALOG))
(COMMON-LISP:LET (#:OUT-914)
(SETQ #:OUT-914 'PRODUCTS)
(COMMON-LISP:LET (#:OUT-913 #:OUT-912)
(SETQ #:OUT-913 (SLOT-VALUE-UNVERSIONED #:OUT-917 #:OUT-914))
(SETQ #:OUT-912
(IF *VERSIONED-VALUE-CID-SET-OVERRIDE*
(PROGN
(DEBUG-MESSAGE 4 "Using override cid-set to scan slot ~s"
#:OUT-914)
*VERSIONED-VALUE-CID-SET-OVERRIDE*)
(TRANSACTION/CID-SET *TRANSACTION*)))
(COMMON-LISP:LET (#:OUT-911 #:OUT-910 #:OUT-909)
(MULTIPLE-VALUE-SETQ (#:OUT-911 #:OUT-910 #:OUT-909)
(CVI-GET-ION-VECTOR-AND-INDEX #:OUT-913 #:OUT-912))
(IF #:OUT-911
NIL
(PROGN
(IF (COMMON-LISP:LET ((#:G717-902 #:OUT-910))
(IF #:G717-902
#:G717-902
(THE T (CVI/DEFAULT-ALLOWED #:OUT-913))))
NIL
(PROGN
(SLOT-UNBOUND (CLASS-OF #:OUT-917) #:OUT-917 #:OUT-914)))))
(DEBUG-MESSAGE 5 "Active ion vector for retrieval:  ~s" #:OUT-911)
(COMMON-LISP:LET (#:OUT-908)
(SETQ #:OUT-908
(IF #:OUT-911
#:OUT-911
(THE T #())))
(COMMON-LISP:LET (#:ELEMENTS-907
(#:LIMIT-905 (ARRAY-TOTAL-SIZE #:OUT-908))
(#:INDEX-904 -1)
(#:INDEX-903 (- -1 1))
#:ITEMS-915
#:ITEMS-900
(#:LASTCONS-897 (LIST NIL))
#:LST-898)
(DECLARE (TYPE SERIES::VECTOR-INDEX+ #:LIMIT-905)
(TYPE SERIES::-VECTOR-INDEX+ #:INDEX-904)
(TYPE INTEGER #:INDEX-903)
(TYPE CONS #:LASTCONS-897)
(TYPE LIST #:LST-898))
(SETQ #:LST-898 #:LASTCONS-897)
(TAGBODY
#:LL-918
(INCF #:INDEX-904)
(LOCALLY
(DECLARE (TYPE SERIES::VECTOR-INDEX+ #:INDEX-904))
(IF (= #:INDEX-904 #:LIMIT-905)
(GO SERIES::END))
(SETQ #:ELEMENTS-907
(ROW-MAJOR-AREF #:OUT-908
(THE SERIES::VECTOR-INDEX
#:INDEX-904))))
(INCF #:INDEX-903)
(IF (MINUSP #:INDEX-903)
(GO #:LL-918))
(SETQ #:ITEMS-915
((LAMBDA (ION-SOUGHT)
(CVI-INSERTION-RECORD/GET-VALUE-FOR-ION
(SVREF #:OUT-909 ION-SOUGHT) ION-SOUGHT))
#:ELEMENTS-907))
(SETQ #:ITEMS-900
((LAMBDA (PRODUCT)
(LIST (DISTRIBUTED-OBJECT-IDENTIFIER PRODUCT)
(NAMED-OBJECT/NAME PRODUCT)
(DESCRIBED-OBJECT/DESCRIPTION PRODUCT)))
#:ITEMS-915))
(SETQ #:LASTCONS-897
(SETF (CDR #:LASTCONS-897) (CONS #:ITEMS-900 NIL)))
(GO #:LL-918)
SERIES::END)
(CDR #:LST-898)))))))```
To be continued...

## Sunday, October 13, 2013

### Next step

We build a simple project/branch/version hierarchical abstraction, and we implement it on top of the core layer.

• a project is a collection of branches that represent alternative ways the state evolves. Every project has a main branch.
• A branch is a collection of versions that represent the evolution of the branch state over time.
• A version is a collection of change-sets with some trappings.
• A change-set is our unit of change.
When we do a read/write transaction, we'll add a new change set to the repository. Read-only transactions will specify a version for reading the core objects. Or two versions for generating diffs. Or maybe even three versions for a three-way merge.

Under this development model, developers will sync their workspace with the most chronologically recent version of the main branch. Their new change sets will be visible only to them, unless (until, we hope) they are "promoted" into the development branch.

We wanted to encourage frequent incremental check-ins. We wanted hook this up to Emacs autosave. Frequent check-ins of much smaller diffs breaks even.

Once you get to the point where your code passes all the tests, you promote it into the branch so everyone can use it. Every now and then the admins will "fork a relase", do some "cherrypicks" and make a release branch in the project.

Once you get to the point where your code passes all the tests, you promote it into the branch so everyone can use it. Every now and then the admins will "fork a relase", do some "cherrypicks" and make a release branch in the project.

The core code does all the heavy lifting, this is window dressing. The style: minimalist. Skin the change model.

This is turning into the code equivalent of a power-point lecture. You can read it yourself, I'm not going to walk you through it.

So that's it? Is that all there is? Many source code or change management systems do something more or less similar to this with files and directory trees instead of CLOS objects. Cute hack, but why all the fuss? Hasn't this been done?

If it seems obvious to you how we'd implement some of the usual source code control operations, good. We don't have to train you.

I wont insult your intelligence explaining in detail how to do a rollback by removing a change-set from a version. Use SETF, of course.

I hope nobody notices the 300 lb chicken sitting next to that shady-looking egg in the corner.

And something for philosopher/implementors to worry about: If I demote the change-set that instantiates an object, where does the object go? Is it possible create a reference to an uninstantiated object? What happens if you try to follow it?