| 1 | (* Copyright (C) 1999-2005 Henry Cejtin, Matthew Fluet, Suresh |
| 2 | * Jagannathan, and Stephen Weeks. |
| 3 | * |
| 4 | * MLton is released under a BSD-style license. |
| 5 | * See the file MLton-LICENSE for details. |
| 6 | *) |
| 7 | |
| 8 | structure BinarySearch: BINARY_SEARCH = |
| 9 | struct |
| 10 | |
| 11 | (* Based on page 38 of Programming Pearls, by Jon Bentley. *) |
| 12 | fun 'a search (a: 'a array, f: 'a -> order): int option = |
| 13 | let |
| 14 | fun loop (min: int, max: int): int option = |
| 15 | if min > max |
| 16 | then NONE |
| 17 | else |
| 18 | let val mid = Int.quot (min + max, 2) |
| 19 | in case f (Array.sub (a, mid)) of |
| 20 | LESS => loop (min, mid - 1) |
| 21 | | EQUAL => SOME mid |
| 22 | | GREATER => loop (mid + 1, max) |
| 23 | end |
| 24 | in loop (0, Array.length a - 1) |
| 25 | end |
| 26 | |
| 27 | fun 'a largest (a: 'a array, f: 'a -> bool): int option = |
| 28 | let |
| 29 | fun loop(min, max, res: int option): int option = |
| 30 | if min > max |
| 31 | then res |
| 32 | else |
| 33 | let val mid = Int.quot(min + max, 2) |
| 34 | in if f(Array.sub(a, mid)) |
| 35 | then loop(mid + 1, max, SOME mid) |
| 36 | else loop(min, mid - 1, res) |
| 37 | end |
| 38 | in loop(0, Array.length a - 1, NONE) |
| 39 | end |
| 40 | |
| 41 | fun 'a smallest(a: 'a array, f: 'a -> bool): int option = |
| 42 | let |
| 43 | fun loop(min, max, res: int option): int option = |
| 44 | if min > max |
| 45 | then res |
| 46 | else |
| 47 | let val mid = Int.quot(min + max, 2) |
| 48 | in if f(Array.sub(a, mid)) |
| 49 | then loop(min, mid - 1, SOME mid) |
| 50 | else loop(mid + 1, max, res) |
| 51 | end |
| 52 | in loop(0, Array.length a - 1, NONE) |
| 53 | end |
| 54 | |
| 55 | end |