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`hEEA_approx_q` as described in https://tches.iacr.org/index.php/TCHES/article/view/12225/12032
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| const std = @import("std"); | |
| const el: u256 = 0x1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed; | |
| const L = 1 * el; | |
| fn heea(v: u256) struct { u128, u128 } { | |
| std.debug.assert(len(L) == 253); | |
| std.debug.assert(v < L); | |
| std.debug.assert(v > 0); | |
| var r0 = L; | |
| var r1 = v; | |
| var t0: u128 = 0; | |
| var t1: u128 = 1; | |
| const stop_value = len(L) / 2 + 1; | |
| var lenr0 = len(L); | |
| var lenr1 = len(v); | |
| while (lenr1 > stop_value) { | |
| const d = lenr0 - lenr1; | |
| var r: u256 = undefined; | |
| var t: u128 = undefined; | |
| if (r0 >> 255 == r1 >> 255) { | |
| r = r0 -% (r1 << @intCast(d)); | |
| t = t0 -% (t1 << @intCast(d)); | |
| } else { | |
| r = r0 +% (r1 << @intCast(d)); | |
| t = t0 +% (t1 << @intCast(d)); | |
| } | |
| if (len(r) > lenr1) { | |
| r0 = r; | |
| t0 = t; | |
| lenr0 = len(r); | |
| } else { | |
| r0 = r1; | |
| r1 = r; | |
| t0 = t1; | |
| t1 = t; | |
| lenr0 = lenr1; | |
| lenr1 = len(r); | |
| } | |
| } | |
| return .{ @truncate(r1), t1 }; | |
| } | |
| fn len(l: u256) u256 { | |
| const normalize = l ^ ((l >> 255) * std.math.maxInt(u256)); | |
| return 256 - @clz(normalize); | |
| } |
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