WIP: Finagle gcd and lcm to satisfy a multiplicative identity

Replicate something similar for radicand analogues.
Fix zero fraction normalization.

ref #27

src/__tests__/fraction-rawify.spec.ts

@@ -989,14 +989,14 @@ const tests = [
     set: -3,
     fn: 'lcm',
     param: 3,
-    expect: '3',
+    expect: '-3', // Deviates from rawify's convention
   },
   {
     label: 'lcm(3,-3)',
     set: 3,
     fn: 'lcm',
     param: -3,
-    expect: '3',
+    expect: '-3', // Deviates from rawify's convention
   },
   {
     label: 'lcm(0,3)',

src/__tests__/fraction.spec.ts

@@ -5,12 +5,49 @@ describe('gcd', () => {
   it('can find the greates common divisor of 12 and 15', () => {
     expect(gcd(12, 15)).toBe(3);
   });
+
+  it('has an identity element (left)', () => {
+    expect(gcd(12, 0)).toBe(12);
+  });
+
+  it('has an identity element (right)', () => {
+    expect(gcd(0, 12)).toBe(12);
+  });
+
+  it('has an identity element (self)', () => {
+    expect(gcd(0, 0)).toBe(0);
+  });
 });
 
 describe('lcm', () => {
   it('can find the least common multiple of 6 and 14', () => {
     expect(lcm(6, 14)).toBe(42);
   });
+
+  it('works with zero (left)', () => {
+    expect(lcm(0, 12)).toBe(0);
+  });
+
+  it('works with zero (right)', () => {
+    expect(lcm(12, 0)).toBe(0);
+  });
+
+  it('works with zero (both)', () => {
+    expect(lcm(0, 0)).toBe(0);
+  });
+});
+
+describe('gcd with lcm', () => {
+  it('satisfies the identity for small integers', () => {
+    for (let i = -10; i <= 10; ++i) {
+      for (let j = -10; j <= 10; ++j) {
+        // We need to bypass (+0).toBe(-0) here...
+        expect(gcd(i, j) * lcm(i, j) === i * j, `failed with ${i}, ${j}`).toBe(
+          true
+        );
+      }
+    }
+  });
 });
 
 describe('mmod', () => {
@@ -303,6 +340,21 @@ describe('Fraction', () => {
     expect(fraction.gcr(Math.random())).toBeNull();
   });
 
+  it('treats unity as the identity if geometric gcd (left)', () => {
+    const fraction = new Fraction(12);
+    expect(fraction.gcr(1)!.equals(12)).toBe(true);
+  });
+
+  it('treats unity as the identity if geometric gcd (right)', () => {
+    const fraction = new Fraction(1);
+    expect(fraction.gcr(12)!.equals(12)).toBe(true);
+  });
+
+  it('treats unity as the identity if geometric gcd (self)', () => {
+    const fraction = new Fraction(1);
+    expect(fraction.gcr(1)!.equals(1)).toBe(true);
+  });
+
   it('has logdivision (integers)', () => {
     const fraction = new Fraction(9);
     expect(fraction.log(3)!.equals(2)).toBe(true);
@@ -356,7 +408,42 @@ describe('Fraction', () => {
 
   it('has a geometric lcm (fractions)', () => {
     const fraction = new Fraction(9, 16);
-    expect(fraction.lcr('64/27')!.equals('4096/729')).toBe(true);
+    // The result for a subunitary argument is subunitary by convention.
+    expect(fraction.lcr('64/27')!.equals('729/4096')).toBe(true);
+  });
+
+  it('satisfies the identity for small integers when it exists', () => {
+    for (let i = 1; i <= 10; ++i) {
+      for (let j = 1; j <= 10; ++j) {
+        const gcr = new Fraction(i).gcr(j);
+        if (gcr === null) {
+          continue;
+        }
+        const lcr = new Fraction(i).lcr(j);
+        expect(lcr!.log(i)!.equals(new Fraction(j).log(gcr)!)).toBe(true);
+      }
+    }
+  });
+
+  it('satisfies the identity between a small integer and a particular when it exists', () => {
+    for (let i = 1; i <= 10; ++i) {
+      const particular = new Fraction(i).inverse();
+      // Starting from 2 to avoid logdivision by unity.
+      for (let j = 2; j <= 10; ++j) {
+        const gcr = particular.gcr(j);
+        if (gcr === null) {
+          continue;
+        }
+        const lcr = particular.lcr(j)!;
+        expect(lcr.log(particular)!.equals(new Fraction(j).log(gcr)!)).toBe(
+          true
+        );
+
+        expect(new Fraction(j).gcr(particular)!.equals(gcr)).toBe(true);
+        expect(new Fraction(j).lcr(particular)!.equals(lcr)).toBe(true);
+        expect(lcr!.log(j)!.equals(particular.log(gcr)!)).toBe(true);
+      }
+    }
   });
 
   it('has geometric rounding (integers)', () => {
@@ -512,4 +599,40 @@ describe('Fraction', () => {
     const b = new Fraction('94906267/987654321');
     expect(a.lcm(b).equals('94906267/9')).toBe(true);
   });
+
+  it('satisfies the multiplicative identity between gcd and lcm for small integers', () => {
+    for (let i = -10; i <= 10; ++i) {
+      for (let j = -10; j <= 10; ++j) {
+        const f = new Fraction(i);
+        expect(
+          f
+            .gcd(j)
+            .mul(f.lcm(j))
+            .equals(i * j),
+          `failed with ${i}, ${j}`
+        ).toBe(true);
+      }
+    }
+  });
+
+  it('normalizes zero (integer)', () => {
+    const fraction = new Fraction(0);
+    expect(fraction.s).toBe(0);
+    expect(fraction.n).toBe(0);
+    expect(fraction.d).toBe(1);
+  });
+
+  it('normalizes zero (numerator)', () => {
+    const fraction = new Fraction({n: -0, d: 1});
+    expect(fraction.s).toBe(0);
+    expect(fraction.n).toBe(0);
+    expect(fraction.d).toBe(1);
+  });
+
+  it('normalizes zero (denominator)', () => {
+    const fraction = new Fraction({n: 0, d: -1});
+    expect(fraction.s).toBe(0);
+    expect(fraction.n).toBe(0);
+    expect(fraction.d).toBe(1);
+  });
 });

src/fraction.ts

@@ -14,6 +14,10 @@ const MAX_CYCLE_LENGTH = 128;
 
 /**
  * Greatest common divisor of two integers.
+ *
+ * Zero is treated as the identity element: gcd(0, x) = gcd(x, 0) = x
+ *
+ * The sign of the result is essentially random for negative inputs.
  * @param a The first integer.
  * @param b The second integer.
  * @returns The largest integer that divides a and b.
@@ -31,12 +35,16 @@ export function gcd(a: number, b: number): number {
 
 /**
  * Least common multiple of two integers.
+ *
+ * Return zero if either of the arguments is zero.
+ *
+ * Satisfies a * b = gcd * lcm. See {@link gcd} for consequences on negative inputs.
  * @param a The first integer.
  * @param b The second integer.
  * @returns The smallest integer that both a and b divide.
  */
 export function lcm(a: number, b: number): number {
-  return (Math.abs(a) / gcd(a, b)) * Math.abs(b);
+  return a ? (a / gcd(a, b)) * b : 0;
 }
 
 /**
@@ -193,11 +201,13 @@ export class Fraction {
       } else {
         this.s = 1;
       }
-      if (numerator.n < 0) {
+      if (numerator.d < 0) {
         this.s = -this.s;
       }
-      if (numerator.d < 0) {
+      if (numerator.n < 0) {
         this.s = -this.s;
+      } else if (numerator.n === 0) {
+        this.s = 0;
       }
       this.n = Math.abs(numerator.n);
       this.d = Math.abs(numerator.d);
@@ -860,6 +870,8 @@ export class Fraction {
   /**
    * Calculates the fractional gcd of two rational numbers. (i.e. both this and other is divisible by the result)
    *
+   * Always returns a non-negative result.
+   *
    * Example:
    * ```ts
    * new Fraction(5,8).gcd("3/7")  // 1/56
@@ -873,17 +885,18 @@ export class Fraction {
   /**
    * Calculates the fractional lcm of two rational numbers. (i.e. the result is divisible by both this and other)
    *
+   * Has the same sign as the product of the rational numbers.
+   *
    * Example:
    * ```ts
    * new Fraction(5,8).gcd("3/7")  // 15
    * ```
    */
   lcm(other: FractionValue) {
-    const {n, d} = new Fraction(other);
-    if (!n && !this.n) {
-      return new Fraction({s: 0, n: 0, d: 1});
-    }
-    return new Fraction(lcm(n, this.n), gcd(d, this.d));
+    const {s, n, d} = new Fraction(other);
+    const result = new Fraction(lcm(n, this.n), gcd(d, this.d));
+    result.s = this.s * s;
+    return result;
   }
 
   /**
@@ -983,6 +996,8 @@ export class Fraction {
   /**
    * Calculate the greatest common radical between two rational numbers if it exists.
    *
+   * Treats unity as the identity element: gcr(1, x) = gcr(x, 1) = x
+   *
    * Examples:
    * ```ts
    * new Fraction(8).gcr(4)          // 2
@@ -1025,6 +1040,14 @@ export class Fraction {
    */
   log(other: FractionValue, maxIter = 100) {
     const other_ = new Fraction(other);
+    if (other_.isUnity()) {
+      if (this.isUnity()) {
+        // This convention follows from an identity between gcr and lcr
+        // Not entirely well-founded, but not entirely wrong either.
+        return new Fraction(1);
+      }
+      return null;
+    }
     const radical = this.gcr(other_, maxIter);
     if (radical === null) {
       return null;
@@ -1057,6 +1080,8 @@ export class Fraction {
   /**
    * Calculate the least common radicand between two rational numbers if it exists.
    *
+   * If either of the inputs is unitary return unity (1).
+   *
    * Examples:
    * ```ts
    * new Fraction(8).lcr(4)          // 64
@@ -1069,10 +1094,13 @@ export class Fraction {
     if (radical === null) {
       return null;
     }
-    const base = 1 / Math.log(radical.n / radical.d);
+    if (radical.isUnity()) {
+      return new Fraction(1);
+    }
+    const base = 1 / Math.abs(Math.log(radical.n / radical.d));
     const n = Math.round(Math.log(this.n / this.d) * base);
     const d = Math.round(Math.log(other_.n / other_.d) * base);
-    return radical.pow(Math.abs(n * d));
+    return radical.pow(n * d);
   }
 
   /**