feat(mf2): Add :math function (unicode-org/message-format-wg#932)

Author: XM5jDcsHTyGJtQqlCi

mf2/messageformat/src/functions/index.ts

@@ -3,6 +3,7 @@ import type { MessageExpressionPart } from '../formatted-parts.js';
 export type { MessageFunctionContext } from '../resolve/function-context.js';
 export { type MessageDateTime, date, datetime, time } from './datetime.js';
 export { type MessageFallback, fallback } from './fallback.js';
+export { math } from './math.js';
 export { type MessageNumber, integer, number } from './number.js';
 export { type MessageString, string } from './string.js';
 export { type MessageUnknownValue, unknown } from './unknown.js';

mf2/messageformat/src/functions/math.ts

@@ -0,0 +1,56 @@
+import { MessageResolutionError } from '../errors.js';
+import type { MessageFunctionContext } from './index.js';
+import { MessageNumber, number } from './number.js';
+import { asPositiveInteger } from './utils.js';
+
+/**
+ * `math` accepts a numeric value as input and adds or subtracts an integer value from it
+ *
+ * @beta
+ */
+export function math(
+  ctx: MessageFunctionContext,
+  mathOpt: Record<string | symbol, unknown>,
+  input?: unknown
+): MessageNumber {
+  const { source } = ctx;
+  let options: unknown = undefined;
+  let value = input;
+  if (typeof value === 'object') {
+    const valueOf = value?.valueOf;
+    if (typeof valueOf === 'function') {
+      options = (value as { options: unknown }).options;
+      value = valueOf.call(value);
+    }
+  }
+  if (typeof value === 'string') {
+    try {
+      value = JSON.parse(value);
+    } catch {
+      // handled below
+    }
+  }
+  if (typeof value !== 'bigint' && typeof value !== 'number') {
+    const msg = 'Input is not numeric';
+    throw new MessageResolutionError('bad-operand', msg, source);
+  }
+
+  let add: number;
+  let sub: number;
+  try {
+    add = 'add' in mathOpt ? asPositiveInteger(mathOpt.add) : -1;
+    sub = 'subtract' in mathOpt ? asPositiveInteger(mathOpt.subtract) : -1;
+  } catch (error) {
+    throw new MessageResolutionError('bad-option', String(error), source);
+  }
+  if (add < 0 === sub < 0) {
+    const msg =
+      'Exactly one of "add" or "subtract" is required as a :math option';
+    throw new MessageResolutionError('bad-option', msg, source);
+  }
+  const delta = add < 0 ? -sub : add;
+  if (typeof value === 'number') value += delta;
+  else value += BigInt(delta);
+
+  return number(ctx, {}, { valueOf: () => value, options });
+}

mf2/messageformat/src/messageformat.ts

@@ -10,6 +10,7 @@ import {
   date,
   datetime,
   integer,
+  math,
   number,
   string,
   time
@@ -24,6 +25,7 @@ const defaultFunctions = Object.freeze({
   date,
   datetime,
   integer,
+  math,
   number,
   string,
   time