75 lines
2.6 KiB
Zig
75 lines
2.6 KiB
Zig
const is_test = @import("builtin").is_test;
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const std = @import("std");
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const math = std.math;
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const Log2Int = std.math.Log2Int;
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const maxInt = std.math.maxInt;
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const minInt = std.math.minInt;
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const DBG = false;
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pub fn fixint(comptime fp_t: type, comptime fixint_t: type, a: fp_t) fixint_t {
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@setRuntimeSafety(is_test);
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const rep_t = switch (fp_t) {
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f32 => u32,
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f64 => u64,
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f128 => u128,
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else => unreachable,
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};
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const significandBits = switch (fp_t) {
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f32 => 23,
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f64 => 52,
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f128 => 112,
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else => unreachable,
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};
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const typeWidth = rep_t.bit_count;
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const exponentBits = (typeWidth - significandBits - 1);
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const signBit = (rep_t(1) << (significandBits + exponentBits));
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const maxExponent = ((1 << exponentBits) - 1);
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const exponentBias = (maxExponent >> 1);
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const implicitBit = (rep_t(1) << significandBits);
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const significandMask = (implicitBit - 1);
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// Break a into sign, exponent, significand
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const aRep: rep_t = @bitCast(rep_t, a);
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const absMask = signBit - 1;
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const aAbs: rep_t = aRep & absMask;
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const negative = (aRep & signBit) != 0;
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const exponent = @intCast(i32, aAbs >> significandBits) - exponentBias;
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const significand: rep_t = (aAbs & significandMask) | implicitBit;
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// If exponent is negative, the uint_result is zero.
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if (exponent < 0) return 0;
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// The unsigned result needs to be large enough to handle an fixint_t or rep_t
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const fixuint_t = @IntType(false, fixint_t.bit_count);
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const UintResultType = if (fixint_t.bit_count > rep_t.bit_count) fixuint_t else rep_t;
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var uint_result: UintResultType = undefined;
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// If the value is too large for the integer type, saturate.
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if (@intCast(usize, exponent) >= fixint_t.bit_count) {
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return if (negative) fixint_t(minInt(fixint_t)) else fixint_t(maxInt(fixint_t));
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}
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// If 0 <= exponent < significandBits, right shift else left shift
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if (exponent < significandBits) {
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uint_result = @intCast(UintResultType, significand) >> @intCast(Log2Int(UintResultType), significandBits - exponent);
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} else {
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uint_result = @intCast(UintResultType, significand) << @intCast(Log2Int(UintResultType), exponent - significandBits);
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}
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// Cast to final signed result
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if (negative) {
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return if (uint_result >= -math.minInt(fixint_t)) math.minInt(fixint_t) else -@intCast(fixint_t, uint_result);
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} else {
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return if (uint_result >= math.maxInt(fixint_t)) math.maxInt(fixint_t) else @intCast(fixint_t, uint_result);
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}
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}
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test "import fixint" {
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_ = @import("fixint_test.zig");
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}
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