From 46680298c88bad12793206a32193bb0f104ce727 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Abel=20Fern=C3=A1ndez?= <abel.fernandez.nandin@gmail.com>
Date: Tue, 15 Sep 2020 10:06:53 +0200
Subject: [PATCH 1/3] Add Decimal.minus() implementation

---
 packages/math/src/decimal.ts | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/packages/math/src/decimal.ts b/packages/math/src/decimal.ts
index ffedd0ed..0d4f1e0a 100644
--- a/packages/math/src/decimal.ts
+++ b/packages/math/src/decimal.ts
@@ -124,6 +124,19 @@ export class Decimal {
     return new Decimal(sum.toString(), this.fractionalDigits);
   }
 
+  /**
+   * a.minus(b) returns a-b.
+   *
+   * Both values need to have the same fractional digits.
+   * The resulting difference needs to be non-negative.
+   */
+  public minus(b: Decimal): Decimal {
+    if (this.fractionalDigits !== b.fractionalDigits) throw new Error("Fractional digits do not match");
+    const difference = this.data.atomics.sub(new BN(b.atomics));
+    if (difference.ltn(0)) throw new Error("Difference must not be negative");
+    return new Decimal(difference.toString(), this.fractionalDigits);
+  }
+
   public equals(b: Decimal): boolean {
     return Decimal.compare(this, b) === 0;
   }

From 0975afcb4a011b0ca3a30c988967547b1e2bc3d7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Abel=20Fern=C3=A1ndez?= <abel.fernandez.nandin@gmail.com>
Date: Tue, 15 Sep 2020 10:07:10 +0200
Subject: [PATCH 2/3] Add Decimal.minus() unit tests

---
 packages/math/src/decimal.spec.ts | 48 +++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/packages/math/src/decimal.spec.ts b/packages/math/src/decimal.spec.ts
index b0e1803b..fee086e9 100644
--- a/packages/math/src/decimal.spec.ts
+++ b/packages/math/src/decimal.spec.ts
@@ -211,6 +211,54 @@ describe("Decimal", () => {
     });
   });
 
+  describe("minus", () => {
+    it("returns correct values", () => {
+      const zero = Decimal.fromUserInput("0", 5);
+      expect(Decimal.fromUserInput("0", 5).minus(zero).toString()).toEqual("0");
+      expect(Decimal.fromUserInput("1", 5).minus(zero).toString()).toEqual("1");
+      expect(Decimal.fromUserInput("2", 5).minus(zero).toString()).toEqual("2");
+      expect(Decimal.fromUserInput("2.8", 5).minus(zero).toString()).toEqual("2.8");
+      expect(Decimal.fromUserInput("0.12345", 5).minus(zero).toString()).toEqual("0.12345");
+
+      const one = Decimal.fromUserInput("1", 5);
+      expect(Decimal.fromUserInput("1", 5).minus(one).toString()).toEqual("0");
+      expect(Decimal.fromUserInput("2", 5).minus(one).toString()).toEqual("1");
+      expect(Decimal.fromUserInput("3", 5).minus(one).toString()).toEqual("2");
+      expect(Decimal.fromUserInput("3.8", 5).minus(one).toString()).toEqual("2.8");
+      expect(Decimal.fromUserInput("1.12345", 5).minus(one).toString()).toEqual("0.12345");
+
+      const oneDotFive = Decimal.fromUserInput("1.5", 5);
+      expect(Decimal.fromUserInput("1.5", 5).minus(oneDotFive).toString()).toEqual("0");
+      expect(Decimal.fromUserInput("2.5", 5).minus(oneDotFive).toString()).toEqual("1");
+      expect(Decimal.fromUserInput("3.5", 5).minus(oneDotFive).toString()).toEqual("2");
+      expect(Decimal.fromUserInput("4.3", 5).minus(oneDotFive).toString()).toEqual("2.8");
+      expect(Decimal.fromUserInput("1.62345", 5).minus(oneDotFive).toString()).toEqual("0.12345");
+
+      // original value remain unchanged
+      expect(zero.toString()).toEqual("0");
+      expect(one.toString()).toEqual("1");
+      expect(oneDotFive.toString()).toEqual("1.5");
+    });
+
+    it("throws for different fractional digits", () => {
+      const zero = Decimal.fromUserInput("0", 5);
+      expect(() => Decimal.fromUserInput("1", 1).minus(zero)).toThrowError(/do not match/i);
+      expect(() => Decimal.fromUserInput("1", 2).minus(zero)).toThrowError(/do not match/i);
+      expect(() => Decimal.fromUserInput("1", 3).minus(zero)).toThrowError(/do not match/i);
+      expect(() => Decimal.fromUserInput("1", 4).minus(zero)).toThrowError(/do not match/i);
+
+      expect(() => Decimal.fromUserInput("1", 6).minus(zero)).toThrowError(/do not match/i);
+      expect(() => Decimal.fromUserInput("1", 7).minus(zero)).toThrowError(/do not match/i);
+    });
+
+    it("throws for negative results", () => {
+      const one = Decimal.fromUserInput("1", 5);
+      expect(() => Decimal.fromUserInput("0", 5).minus(one)).toThrowError(/must not be negative/i);
+      expect(() => Decimal.fromUserInput("0.5", 5).minus(one)).toThrowError(/must not be negative/i);
+      expect(() => Decimal.fromUserInput("0.98765", 5).minus(one)).toThrowError(/must not be negative/i);
+    });
+  });
+
   describe("equals", () => {
     it("returns correct values", () => {
       const zero = Decimal.fromUserInput("0", 5);

From eeec000076c63db51a01515f7dea4a737bcf7bd0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Abel=20Fern=C3=A1ndez?= <abel.fernandez.nandin@gmail.com>
Date: Tue, 15 Sep 2020 10:08:00 +0200
Subject: [PATCH 3/3] Add Decimal.minus() to declaration file

---
 packages/math/types/decimal.d.ts | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/packages/math/types/decimal.d.ts b/packages/math/types/decimal.d.ts
index 0642fa07..123da718 100644
--- a/packages/math/types/decimal.d.ts
+++ b/packages/math/types/decimal.d.ts
@@ -24,6 +24,13 @@ export declare class Decimal {
    * Both values need to have the same fractional digits.
    */
   plus(b: Decimal): Decimal;
+  /**
+   * a.minus(b) returns a-b.
+   *
+   * Both values need to have the same fractional digits.
+   * The resulting difference needs to be non-negative.
+   */
+  minus(b: Decimal): Decimal;
   equals(b: Decimal): boolean;
   isLessThan(b: Decimal): boolean;
   isLessThanOrEqual(b: Decimal): boolean;