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Polynomial rolling hash mod 1000003 with base 257: - precompute base^(m-1) - slide window updating hash in O(1) per step - verify hash match with O(m) memcmp to skip false positives rolling_match "abcabcabcabcabcabc" "abc" = 6 Six non-overlapping copies of "abc" at positions 0,3,6,9,12,15. Tests `for _ = 0 to m - 2 do … done` unused loop variable (uses underscore wildcard pattern), Char.code arithmetic, mod arithmetic with intermediate negative subtractions, complex nested if/begin branching with inner break-via-flag. 151 baseline programs total.
39 lines
1022 B
OCaml
39 lines
1022 B
OCaml
let mod_p = 1000003
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let base = 257
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let rolling_match text pat =
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let n = String.length text in
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let m = String.length pat in
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if m > n then 0
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else begin
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let pat_hash = ref 0 in
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let win_hash = ref 0 in
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let high = ref 1 in
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for _ = 0 to m - 2 do
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high := (!high * base) mod mod_p
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done;
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for i = 0 to m - 1 do
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pat_hash := (!pat_hash * base + Char.code pat.[i]) mod mod_p;
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win_hash := (!win_hash * base + Char.code text.[i]) mod mod_p
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done;
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let count = ref 0 in
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for i = 0 to n - m do
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if !win_hash = !pat_hash then begin
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let ok = ref true in
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for j = 0 to m - 1 do
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if text.[i + j] <> pat.[j] then ok := false
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done;
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if !ok then count := !count + 1
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end;
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if i < n - m then begin
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let drop = (Char.code text.[i] * !high) mod mod_p in
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win_hash := ((!win_hash - drop + mod_p) * base + Char.code text.[i + m]) mod mod_p
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end
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done;
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!count
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end
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;;
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rolling_match "abcabcabcabcabcabc" "abc"
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