ocaml: phase 5.1 lev_iter.ml baseline (sum of 5 edit distances = 16)

Iterative Levenshtein DP with rolling 1D arrays for O(min(m,n))
space. Distances:

  kitten    -> sitting    : 3
  saturday  -> sunday     : 3
  abc       -> abc        : 0
  ""        -> abcde      : 5
  intention -> execution  : 5
  ----------------------------
  total                   : 16

Complementary to the existing levenshtein.ml which uses the
exponential recursive form (only sums tiny strings); this one is
the practical iterative variant used for real ED.

Tests the recently-fixed <- with bare `if` rhs:

  curr.(j) <- (if m1 < c then m1 else c) + 1

153 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -90,6 +90,7 @@
   "lcs.ml": 4,
   "majority_vote.ml": 4,
   "manacher.ml": 7,
+  "lev_iter.ml": 16,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
   "mortgage.ml": 1073,

lib/ocaml/baseline/lev_iter.ml

@@ -0,0 +1,30 @@
+let lev_iter s1 s2 =
+  let m = String.length s1 in
+  let n = String.length s2 in
+  let prev = Array.make (n + 1) 0 in
+  let curr = Array.make (n + 1) 0 in
+  for j = 0 to n do prev.(j) <- j done;
+  for i = 1 to m do
+    curr.(0) <- i;
+    for j = 1 to n do
+      if s1.[i - 1] = s2.[j - 1] then
+        curr.(j) <- prev.(j - 1)
+      else begin
+        let a = prev.(j) in
+        let b = curr.(j - 1) in
+        let c = prev.(j - 1) in
+        let m1 = if a < b then a else b in
+        curr.(j) <- (if m1 < c then m1 else c) + 1
+      end
+    done;
+    for j = 0 to n do prev.(j) <- curr.(j) done
+  done;
+  prev.(n)
+
+;;
+
+lev_iter "kitten" "sitting"
++ lev_iter "saturday" "sunday"
++ lev_iter "abc" "abc"
++ lev_iter "" "abcde"
++ lev_iter "intention" "execution"