feat: port rust RocDec.mul logic over to zig

compiler/builtins/bitcode/src/dec.zig

@@ -1,8 +1,17 @@
 const std = @import("std");
 
+const math = std.math;
+
 pub const RocDec = struct {
     num: i128,
 
+    pub const decimal_places: u32 = 18;
+
+    pub const min: RocDec = .{ .num = math.minInt(i128) };
+    pub const max: RocDec = .{ .num = math.maxInt(i128) };
+
+    pub const one_point_zero: RocDec = .{ .num = comptime math.pow(i128, 10, RocDec.decimal_places) };
+
     pub fn add(self: RocDec, other: RocDec) RocDec {
         var answer: i128 = undefined;
         const overflowed = @addWithOverflow(i128, self.num, other.num, &answer);
@@ -15,44 +24,214 @@ pub const RocDec = struct {
     }
 
     pub fn mul(self: RocDec, other: RocDec) RocDec {
-        const self_i256 = @intCast(i256, self.num);
+        const self_i128 = self.num;
-        const other_i256 = @intCast(i256, other.num);
+        const other_i128 = other.num;
-        const answer = 0; //self_i256 * other_i256;
+        // const answer = 0; //self_i256 * other_i256;
 
-        if ((answer >> 192) < 0) {
+        const is_answer_negative = (self_i128 < 0) != (other_i128 < 0);
-            return RocDec{ .num = 0 };
+
-            // return RocDec{ .num = @intCast(i128, @divTrunc(answer, one_e20)) };
+        const self_u128 = @intCast(u128, math.absInt(self_i128) catch {
+            if (other_i128 == 0) {
+                return .{ .num = 0 };
+            } else if (other_i128 == RocDec.one_point_zero.num) {
+                return self;
             } else {
-            return RocDec{ .num = 0 };
+                std.debug.panic("TODO runtime exception for overflow!", .{});
-           // std.debug.panic("TODO runtime exception for overflow!", .{});
+            }
+        });
+
+        const other_u128 = @intCast(u128, math.absInt(other_i128) catch {
+            if (self_i128 == 0) {
+                return .{ .num = 0 };
+            } else if (self_i128 == RocDec.one_point_zero.num) {
+                return other;
+            } else {
+                std.debug.panic("TODO runtime exception for overflow!", .{});
+            }
+        });
+
+        const unsigned_answer: i128 = mul_and_decimalize(self_u128, other_u128);
+
+        if (is_answer_negative) {
+            return .{ .num = -unsigned_answer };
+        } else {
+            return .{ .num = unsigned_answer };
         }
     }
 };
 
+const U256 = struct {
+    hi: u128,
+    lo: u128,
+};
+
+fn mul_and_decimalize(a: u128, b: u128) i128 {
+    const answer_u256 = mul_u128(a, b);
+
+    var lhs_hi = answer_u256.hi;
+    var lhs_lo = answer_u256.lo;
+
+    // Divide - or just add 1, multiply by floor(2^315/10^18), then right shift 315 times.
+    // floor(2^315/10^18) is 66749594872528440074844428317798503581334516323645399060845050244444366430645
+
+    // Add 1.
+    // This can't overflow because the intial numbers are only 127bit due to removing the sign bit.
+    var overflowed = @addWithOverflow(u128, lhs_lo, 1, &lhs_lo);
+    lhs_hi = blk: {
+        if (overflowed) {
+            break :blk lhs_hi + 1;
+        } else {
+            break :blk lhs_hi + 0;
+        }
+    };
+
+    // This needs to do multiplication in a way that expands,
+    // since we throw away 315 bits we care only about the higher end, not lower.
+    // So like need to do high low mult with 2 U256's and then bitshift.
+    // I bet this has a lot of room for multiplication optimization.
+    const rhs_hi: u128 = 0x9392ee8e921d5d073aff322e62439fcf;
+    const rhs_lo: u128 = 0x32d7f344649470f90cac0c573bf9e1b5;
+
+    const ea = mul_u128(lhs_lo, rhs_lo);
+    const gf = mul_u128(lhs_hi, rhs_lo);
+    const jh = mul_u128(lhs_lo, rhs_hi);
+    const lk = mul_u128(lhs_hi, rhs_hi);
+
+    const e = ea.hi;
+    const _a = ea.lo;
+
+    const g = gf.hi;
+    const f = gf.lo;
+
+    const j = jh.hi;
+    const h = jh.lo;
+
+    const l = lk.hi;
+    const k = lk.lo;
+
+    // b = e + f + h
+    var e_plus_f: u128 = undefined;
+    overflowed = @addWithOverflow(u128, e, f, &e_plus_f);
+    var b_carry1: u128 = undefined;
+    if (overflowed) {
+        b_carry1 = 1;
+    } else {
+        b_carry1 = 0;
+    }
+
+    var idk: u128 = undefined;
+    overflowed = @addWithOverflow(u128, e_plus_f, h, &idk);
+    var b_carry2: u128 = undefined;
+    if (overflowed) {
+        b_carry2 = 1;
+    } else {
+        b_carry2 = 0;
+    }
+
+    // c = carry + g + j + k // it doesn't say +k but I think it should be?
+    var g_plus_j: u128 = undefined;
+    overflowed = @addWithOverflow(u128, g, j, &g_plus_j);
+    var c_carry1: u128 = undefined;
+    if (overflowed) {
+        c_carry1 = 1;
+    } else {
+        c_carry1 = 0;
+    }
+
+    var g_plus_j_plus_k: u128 = undefined;
+    overflowed = @addWithOverflow(u128, g_plus_j, k, &g_plus_j_plus_k);
+    var c_carry2: u128 = undefined;
+    if (overflowed) {
+        c_carry2 = 1;
+    } else {
+        c_carry2 = 0;
+    }
+
+    var c_without_bcarry2: u128 = undefined;
+    overflowed = @addWithOverflow(u128, g_plus_j_plus_k, b_carry1, &c_without_bcarry2);
+    var c_carry3: u128 = undefined;
+    if (overflowed) {
+        c_carry3 = 1;
+    } else {
+        c_carry3 = 0;
+    }
+
+    var c: u128 = undefined;
+    overflowed = @addWithOverflow(u128, c_without_bcarry2, b_carry2, &c);
+    var c_carry4: u128 = undefined;
+    if (overflowed) {
+        c_carry4 = 1;
+    } else {
+        c_carry4 = 0;
+    }
+
+    // d = carry + l
+    var d: u128 = undefined;
+    overflowed = @addWithOverflow(u128, l, c_carry1, &d);
+    overflowed = overflowed or @addWithOverflow(u128, d, c_carry2, &d);
+    overflowed = overflowed or @addWithOverflow(u128, d, c_carry3, &d);
+    overflowed = overflowed or @addWithOverflow(u128, d, c_carry4, &d);
+
+    if (overflowed) {
+        std.debug.panic("TODO runtime exception for overflow!", .{});
+    }
+
+    // Final 512bit value is d, c, b, a
+    // need to left shift 321 times
+    // 315 - 256 is 59. So left shift d, c 59 times.
+    return @intCast(i128, c >> 59 | (d << (128 - 59)));
+}
+
+fn mul_u128(a: u128, b: u128) U256 {
+    var hi: u128 = undefined;
+    var lo: u128 = undefined;
+
+    const bits_in_dword_2: u32 = 64;
+    const lower_mask: u128 = math.maxInt(u128) >> bits_in_dword_2;
+
+    lo = (a & lower_mask) * (b & lower_mask);
+
+    var t = lo >> bits_in_dword_2;
+
+    lo &= lower_mask;
+
+    t += (a >> bits_in_dword_2) * (b & lower_mask);
+
+    lo += (t & lower_mask) << bits_in_dword_2;
+
+    hi = t >> bits_in_dword_2;
+
+    t = lo >> bits_in_dword_2;
+
+    lo &= lower_mask;
+
+    t += (b >> bits_in_dword_2) * (a & lower_mask);
+
+    lo += (t & lower_mask) << bits_in_dword_2;
+
+    hi += t >> bits_in_dword_2;
+
+    hi += (a >> bits_in_dword_2) * (b >> bits_in_dword_2);
+
+    return .{ .hi = hi, .lo = lo };
+}
+
 const one_e20: i256 = 100000000000000000000;
 
 const expectEqual = std.testing.expectEqual;
 
 test "add" {
-    expectEqual(
+    const dec: RocDec = .{ .num = 0 };
-        RocDec { .num = 0 },
+
-        (RocDec{ .num = 0 }).add(RocDec{ .num = 0 })
+    expectEqual(RocDec{ .num = 0 }, dec.add(.{ .num = 0 }));
-    );
 }
 
 test "mul" {
-    expectEqual(
+    var dec1: RocDec = .{ .num = 0 };
-        RocDec { .num = 0 },
-        (RocDec{ .num = 0 }).mul(RocDec{ .num = 0 })
-    );
 
-    expectEqual(
+    expectEqual(RocDec{ .num = 0 }, dec1.mul(.{ .num = 0 }));
-        RocDec { .num = 100000000000000000000 },
-        (RocDec{ .num = 100000000000000000000 }).mul(RocDec{ .num = 100000000000000000000 })
-    );
 
-    // expectEqual(
+    var dec2: RocDec = .{ .num = 100000000000000000000 };
-    //     (RocDec{ .num = 1 }).add(RocDec{ .num = 1 }),
+
-    //     RocDec { .num = 0 }
+    expectEqual(RocDec{ .num = 100000000000000000000 }, dec2.mul(.{ .num = 100000000000000000000 }));
-    // );
 }

compiler/builtins/bitcode/src/main.zig

@@ -2,6 +2,9 @@ const builtin = @import("builtin");
 const std = @import("std");
 const testing = std.testing;
 
+// Dec Module
+const dec = @import("dec.zig");
+
 // List Module
 const list = @import("list.zig");