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d493f6696b
The implementation uses multiply-then-XOR order, which is FNV-1. FNV-1a would XOR first, then multiply. The constants (offset_basis and prime) are the same for both variants; only the operation order differs. Fix comments in cc_stubs.c, CCInt.ml, and CCHash.ml. No behavioral change — just correcting the documentation.
53 lines
1.7 KiB
C
53 lines
1.7 KiB
C
#include <caml/mlvalues.h>
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#include <stdint.h>
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/* FNV-1 hash for a 64-bit integer.
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https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
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FNV-1 order: multiply then XOR (as opposed to FNV-1a which XORs first).
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Uses the standard 64-bit FNV parameters:
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offset_basis = 0xcbf29ce484222325
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prime = 0x00000100000001b3
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Core routine: operates on all 8 bytes of an int64_t. */
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static inline int64_t cc_fnv_hash_int64(int64_t n) {
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uint64_t un = (uint64_t)n;
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uint64_t h = UINT64_C(0xcbf29ce484222325);
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const uint64_t prime = UINT64_C(0x100000001b3);
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for (int k = 0; k < 8; k++) {
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h *= prime;
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h ^= (un >> (k * 8)) & 0xff;
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}
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return (int64_t)h;
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}
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/* --- CCInt.hash entry points (int -> int) --- */
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/* Mask to the OCaml int range (63 bits on 64-bit, 31 on 32-bit)
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before hashing, so negative OCaml ints hash the same as
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the unsigned representation seen by OCaml's [lsr]. */
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#define OCAML_INT_MASK ((UINT64_C(1) << (8 * sizeof(value) - 1)) - 1)
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/* native: untagged int in, untagged int out */
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CAMLprim intnat caml_cc_hash_int(intnat n) {
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int64_t projected = (int64_t)((uint64_t)n & OCAML_INT_MASK);
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return (intnat)((uint64_t)cc_fnv_hash_int64(projected) & Max_long);
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}
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/* bytecode: boxed value in, boxed value out */
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CAMLprim value caml_cc_hash_int_byte(value v_n) {
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return Val_long(caml_cc_hash_int(Long_val(v_n)));
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}
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/* --- int64 hash entry points (int64 -> int) --- */
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/* native: unboxed int64 in, untagged int out */
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CAMLprim intnat caml_cc_hash_int64(int64_t n) {
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return (intnat)((uint64_t)cc_fnv_hash_int64(n) & Max_long);
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}
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/* bytecode: boxed int64 value in, boxed value out */
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CAMLprim value caml_cc_hash_int64_byte(value v_n) {
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return Val_long(caml_cc_hash_int64(Int64_val(v_n)));
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}
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