Preserve integral precision when parsing decimal Duration inputs

.changeset/fix-duration-decimal-precision.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Preserve integral precision when parsing decimal nano and micro duration inputs

packages/effect/src/Duration.ts

@@ -27,6 +27,8 @@ const TypeId = "~effect/time/Duration"
 
 const bigint0 = BigInt(0)
 const bigint1 = BigInt(1)
+const bigint2 = BigInt(2)
+const bigint10 = BigInt(10)
 const bigint24 = BigInt(24)
 const bigint60 = BigInt(60)
 const bigint1e3 = BigInt(1_000)
@@ -38,8 +40,20 @@ const roundTiesAwayFromZero = (input: number): bigint =>
 
 const roundMillisToNanos = (millis: number): bigint => roundTiesAwayFromZero(millis * 1_000_000)
 
-const parseNanos = (input: string, scale: bigint): bigint =>
-  input.includes(".") ? roundTiesAwayFromZero(Number(input) * Number(scale)) : BigInt(input) * scale
+const parseNanos = (input: string, scale: bigint): bigint => {
+  const decimalIndex = input.indexOf(".")
+  if (decimalIndex === -1) return BigInt(input) * scale
+
+  const isNegative = input[0] === "-"
+  const fractional = input.slice(decimalIndex + 1)
+  const fractionalScale = bigint10 ** BigInt(fractional.length)
+  const scaled = (
+    BigInt(input.slice(isNegative ? 1 : 0, decimalIndex)) * fractionalScale + BigInt(fractional)
+  ) * scale
+  const rounded = scaled / fractionalScale +
+    (scaled % fractionalScale * bigint2 >= fractionalScale ? bigint1 : bigint0)
+  return isNegative ? -rounded : rounded
+}
 
 const nanosToHrTime = (nanos: bigint): [seconds: number, nanos: number] => {
   const sign = nanos < bigint0 ? -bigint1 : bigint1

packages/effect/test/Duration.test.ts

@@ -23,10 +23,20 @@ describe("Duration", () => {
     deepStrictEqual(Duration.fromInputUnsafe("10 nanos"), Duration.nanos(10n))
     deepStrictEqual(Duration.fromInputUnsafe("1.5 nanos"), Duration.nanos(2n))
     deepStrictEqual(Duration.fromInputUnsafe("-1.5 nanos"), Duration.nanos(-2n))
+    deepStrictEqual(Duration.fromInputUnsafe("9007199254740993.1 nanos"), Duration.nanos(9_007_199_254_740_993n))
+    deepStrictEqual(Duration.fromInputUnsafe("-9007199254740993.5 nanos"), Duration.nanos(-9_007_199_254_740_994n))
     deepStrictEqual(Duration.fromInputUnsafe("1 micro"), Duration.micros(1n))
     deepStrictEqual(Duration.fromInputUnsafe("10 micros"), Duration.micros(10n))
     deepStrictEqual(Duration.fromInputUnsafe("1.5 micros"), Duration.nanos(1500n))
     deepStrictEqual(Duration.fromInputUnsafe("-1.5 micros"), Duration.nanos(-1500n))
+    deepStrictEqual(
+      Duration.fromInputUnsafe("9007199254740993.1 micros"),
+      Duration.nanos(9_007_199_254_740_993_100n)
+    )
+    deepStrictEqual(
+      Duration.fromInputUnsafe("-9007199254740993.0005 micros"),
+      Duration.nanos(-9_007_199_254_740_993_001n)
+    )
     deepStrictEqual(Duration.fromInputUnsafe("1 milli"), Duration.millis(1))
     deepStrictEqual(Duration.fromInputUnsafe("10 millis"), Duration.millis(10))
     deepStrictEqual(Duration.fromInputUnsafe("1 second"), Duration.seconds(1))