user1287161 user1287161 - 1 year ago 87
C Question

xxhash function is trying to add more than uint32 max?

I don't understand how this xxhash function is able to add larger values than uint32 can store. Can someone explain why this wizardry is possible?

static const U32 PRIME32_1 = 2654435761U;
static const U32 PRIME32_2 = 2246822519U;
U32 v1 = seed + PRIME32_1 + PRIME32_2;

Answer Source

The standard requires that the standard unsigned integral types handle overflow/underflow by 'wrapping' the value modulo FOO_MAX + 1. (e.g. UINT32_MAX is the macro holding the maximum value of uint32_t)

Assuming U32 refers to uint32_t, the additions are performed modulo 4294967296.

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