pure TS amount arithmetic

extension/lib/wallet/types.ts

@@ -126,6 +126,91 @@ export interface Reserve {
   reserve_pub: string;
 }
 
+export namespace Amounts {
+  export interface Result {
+    amount: AmountJson;
+    // Was there an over-/underflow?
+    saturated: boolean;
+  }
+
+  function getMaxAmount(currency: string) {
+    return {
+      currency,
+      value: Number.MAX_SAFE_INTEGER,
+      fraction: 2**32,
+    }
+  }
+
+  export function add(first: AmountJson, ...rest: AmountJson[]): Result {
+    let currency = first.currency;
+    let value = first.value + Math.floor(first.fraction / 1e6);
+    if (value > Number.MAX_SAFE_INTEGER) {
+      return {amount: getMaxAmount(currency), saturated: true};
+    }
+    let fraction = first.fraction;
+    for (let x of rest) {
+      if (x.currency !== currency) {
+        throw Error(`Mismatched currency: ${x.currency} and ${currency}`);
+      }
+
+      fraction = (fraction + x.fraction) % 1e6;
+      value = value + x.value + (Math.floor(fraction + x.fraction) / 1e6);
+      if (value > Number.MAX_SAFE_INTEGER) {
+        return {amount: getMaxAmount(currency), saturated: true};
+      }
+    }
+    return {amount: {currency, value, fraction}, saturated: false};
+  }
+
+
+  export function sub(a: AmountJson, b: AmountJson): Result {
+    if (a.currency !== b.currency) {
+      throw Error(`Mismatched currency: ${a.currency} and ${b.currency}`);
+    }
+    let currency = a.currency;
+    let value = a.value;
+    let fraction = a.fraction;
+    if (fraction < b.fraction) {
+      if (value < 1) {
+        return {amount: {currency, value: 0, fraction: 0}, saturated: true};
+      }
+      value--;
+      fraction = +1e6;
+    }
+    console.assert(fraction >= b.fraction);
+    fraction -= b.fraction;
+    if (value < b.value) {
+      return {amount: {currency, value: 0, fraction: 0}, saturated: true};
+    }
+    value -= b.value;
+    return {amount: {currency, value, fraction}, saturated: false};
+  }
+
+  export function cmp(a: AmountJson, b: AmountJson): number {
+    if (a.currency !== b.currency) {
+      throw Error(`Mismatched currency: ${a.currency} and ${b.currency}`);
+    }
+    let av = a.value + Math.floor(a.fraction / 1e6);
+    let af = a.fraction % 1e6;
+    let bv = b.value + Math.floor(b.fraction / 1e6);
+    let bf = b.fraction % 1e6;
+    switch (true) {
+      case av < bv:
+        return -1;
+      case av > bv:
+        return 1;
+      case af < bf:
+        return -1;
+      case af > bf:
+        return 1;
+      case af == bf:
+        return 0;
+      default:
+        throw Error("assertion failed");
+    }
+  }
+}
+
 export interface Notifier {
   notify();
 }