Add tests for toPrecision

Author: jessealama

src/decimal128.mts

@@ -950,8 +950,66 @@ export class Decimal128 {
         return this.round(n).emitDecimal(opts);
     }
 
-    toPrecision(n?: number): string {
-        return "6";
+    toPrecision(opts?: { digits?: number }): string {
+        if (undefined === opts || undefined === opts.digits) {
+            return this.toString();
+        }
+
+        let n = opts.digits;
+
+        if (n <= 0) {
+            throw new RangeError("Argument must be positive");
+        }
+
+        if (!Number.isInteger(n)) {
+            throw new RangeError("Argument must be an integer");
+        }
+
+        if (this.isNaN) {
+            return "NaN";
+        }
+
+        if (!this.isFinite) {
+            return (this.isNegative ? "-" : "") + "Infinity";
+        }
+
+        let s = this.abs().emitDecimal({
+            normalize: false,
+            numDecimalDigits: undefined,
+            format: "decimal",
+        });
+
+        let [lhs, rhs] = s.split(/[.]/);
+        let p = this.isNegative ? "-" : "";
+
+        if (n <= lhs.length) {
+            if (lhs.match(/[.]$/)) {
+                lhs = lhs.substring(0, n);
+            }
+
+            if (lhs.length === n) {
+                return p + lhs;
+            }
+
+            if (1 === n) {
+                return p + s.substring(0, 1) + "e+" + `${lhs.length - n}`;
+            }
+
+            return (
+                p +
+                s.substring(0, 1) +
+                "." +
+                s.substring(1, n) +
+                "e+" +
+                `${lhs.length - n + 1}`
+            );
+        }
+
+        if (n <= lhs.length + rhs.length) {
+            return p + s.substring(0, n + 1); // plus one because of the decimal point
+        }
+
+        return p + lhs + "." + rhs + "0".repeat(n - lhs.length - rhs.length);
     }
 
     toExponential(): string {

tests/tofixed.test.js

@@ -36,10 +36,10 @@ describe("to decimal places", function () {
     test("same number of digits as available means no change", () => {
         expectDecimal128(decimalD.toFixed(3), "123.456");
     });
-    test("cutoff if number has more digits than requested (1)", () => {
-        expectDecimal128(decimalD.toFixed(2), "123.45");
+    test("cutoff with rounding if number has more digits than requested (1)", () => {
+        expectDecimal128(decimalD.toFixed(2), "123.46");
     });
-    test("cutoff if number has more digits than requested (2)", () => {
+    test("cutoff if number has more digits than requested (no rounding)", () => {
         expectDecimal128(decimalD.toFixed(1), "123.4");
     });
     test("zero decimal places", () => {

tests/toprecison.test.js

@@ -29,35 +29,57 @@ describe("infinity", () => {
     });
 });
 
-describe("to decimal places", function () {
+describe("toprecision", function () {
     const d = "123.456";
     const decimalD = new Decimal128(d);
-    test("more digits than available means digits get added", () => {
-        expectDecimal128(
-            decimalD.toString({ numDecimalDigits: 4 }),
-            "123.4560"
-        );
+    test("no argument", () => {
+        expect(decimalD.toPrecision()).toStrictEqual("123.456");
+    });
+    test("wrong argument type", () => {
+        expect(decimalD.toPrecision("foo")).toStrictEqual("123.456");
+    });
+    test("empty options", () => {
+        expect(decimalD.toPrecision({})).toStrictEqual("123.456");
+    });
+    test("expected property missing", () => {
+        expect(decimalD.toPrecision({ foo: "bar" })).toStrictEqual("123.456");
+    });
+    test("more digits requested than integer digits available", () => {
+        expectDecimal128(decimalD.toPrecision({ digits: 7 }), "123.4560");
+    });
+    test("exact number of digits requested as digits available", () => {
+        expectDecimal128(decimalD.toPrecision({ digits: 6 }), "123.456");
+    });
+    test("possibly round non-integer part (1)", () => {
+        expectDecimal128(decimalD.toPrecision({ digits: 5 }), "123.45");
+    });
+    test("possibly round non-integer part (2)", () => {
+        expectDecimal128(decimalD.toPrecision({ digits: 4 }), "123.4");
     });
     test("same number of digits as available means no change", () => {
-        expectDecimal128(decimalD.toString({ numDecimalDigits: 3 }), "123.456");
+        expectDecimal128(decimalD.toPrecision({ digits: 3 }), "123");
     });
     test("cutoff if number has more digits than requested (1)", () => {
-        expectDecimal128(decimalD.toString({ numDecimalDigits: 2 }), "123.45");
+        expectDecimal128(decimalD.toPrecision({ digits: 2 }), "1.2e+2");
     });
     test("cutoff if number has more digits than requested (2)", () => {
-        expectDecimal128(decimalD.toString({ numDecimalDigits: 1 }), "123.4");
+        expectDecimal128(decimalD.toPrecision({ digits: 1 }), "1e+2");
     });
-    test("zero decimal places", () => {
-        expectDecimal128(decimalD.toString({ numDecimalDigits: 0 }), "123");
+    test("zero decimal places throws", () => {
+        expect(() => decimalD.toPrecision({ digits: 0 })).toThrow(RangeError);
     });
     test("negative number of decimal places", () => {
-        expect(decimalD.toString({ numDecimalDigits: -1 })).toStrictEqual(
-            "123.456"
-        );
+        expect(() => decimalD.toPrecision({ digits: -1 })).toThrow(RangeError);
     });
-    test("non-integer number of decimal places reverts to default", () => {
-        expect(decimalD.toString({ numDecimalDigits: 1.5 })).toStrictEqual(
-            "123.456"
-        );
+    test("non-integer number throws", () => {
+        expect(() => decimalD.toPrecision({ digits: 1.5 })).toThrow(RangeError);
+    });
+    describe("negative", () => {
+        let negD = decimalD.neg();
+        test("integer part", () => {
+            expect(negD.toPrecision({ digits: 3 }).toString()).toStrictEqual(
+                "-123"
+            );
+        });
     });
 });