From 62593b0ab8dfaa63a788702762146803c9845a98 Mon Sep 17 00:00:00 2001
From: Tangent Wantwight <tangent128@gmail.com>
Date: Fri, 17 May 2024 20:26:54 -0400
Subject: [PATCH] Start stubbing out Pratt parser - parse integers

---
 src/lib/expr.test.ts | 23 +++++++++++
 src/lib/expr.ts      | 94 ++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 113 insertions(+), 4 deletions(-)
 create mode 100644 src/lib/expr.test.ts

diff --git a/src/lib/expr.test.ts b/src/lib/expr.test.ts
new file mode 100644
index 0000000..eea76f7
--- /dev/null
+++ b/src/lib/expr.test.ts
@@ -0,0 +1,23 @@
+import { AsText, TextPiece } from "../words";
+import { Expr } from "./expr";
+
+describe("expr", () => {
+  test.each([
+    ["1", "1"],
+    // ["1 + 2", "3"],
+    // ["1 - 2", "-1"],
+    // ["1 * 2", "2"],
+    // ["1 / 2", "0.5"],
+    // ["1 // 2", "0"],
+    // TODO: operator precedence
+    // TODO: parentheses
+    // TODO; eror reporting
+  ])("Expr does math: %s", (expression, result) => {
+    const actualResult = Expr({}, [{ text: expression }]);
+    console.log(actualResult)
+    expect("error" in actualResult).toBeFalsy();
+    expect(AsText(actualResult as TextPiece)).toEqual(result);
+  });
+
+  // TODO: handle expr prefix
+});
diff --git a/src/lib/expr.ts b/src/lib/expr.ts
index 59e01f3..27cc2fe 100644
--- a/src/lib/expr.ts
+++ b/src/lib/expr.ts
@@ -1,4 +1,4 @@
-import { AsText, ProcResult, TextPiece } from "../words";
+import { AsText, ErrorResult, ProcResult, TextPiece } from "../words";
 
 export function Expr({}, argv: TextPiece[]): ProcResult {
   const name = argv[0];
@@ -6,7 +6,93 @@ export function Expr({}, argv: TextPiece[]): ProcResult {
     // being called as [expr ...], not fallback for math
     argv.splice(0, 1);
   }
-  return {
-    text: `Will do math to solve ${argv.map(AsText).join(" ")} eventually`,
-  };
+
+  const expression = argv.map(AsText).join(" ").trim();
+
+  const parser = new ExpressionParser(expression);
+  const result = parser.parsePrefixOp();
+
+  if ("value" in result) {
+    return { text: String(result.value) };
+  } else {
+    return result;
+  }
 }
+
+// ---------------------------
+
+// Pratt parser for evaluating arithmatic expressions
+
+const PLUS_TOKEN = /\s*(\+)/y;
+const MINUS_TOKEN = /\s*(\-)/y;
+const TIMES_TOKEN = /\s*(\*)/y;
+const FLOOR_TOKEN = /\s*(\/\/)/y;
+const DIV_TOKEN = /\s*(\/)/y;
+
+const NUMBER_TOKEN = /\s*(\d+)/y;
+
+type Value = { value: number };
+
+type TokenHandler = {
+  leftBindingPower: number;
+  token: RegExp;
+  parse: (matched: string, parser: ExpressionParser) => Value | ErrorResult;
+};
+
+const Operators: TokenHandler[] = [
+  {
+    leftBindingPower: -1,
+    token: NUMBER_TOKEN,
+    parse: (matched) => ({ value: Number(matched) }),
+  },
+];
+
+class ExpressionParser {
+  pos: number = 0;
+
+  constructor(public text: string) {}
+
+  next(token: RegExp): string | null {
+    token.lastIndex = this.pos;
+    const matches = token.exec(this.text);
+    if (matches) {
+      this.pos = token.lastIndex;
+      return matches[1];
+    } else {
+      return null;
+    }
+  }
+
+  /** Tries to match one of the given tokens & returns the value of the handler. If none match, returns null. */
+  tryAll(options: TokenHandler[]): Value | ErrorResult | null {
+    for (const option of options) {
+      const match = this.next(option.token);
+      if (match) {
+        return option.parse(match, this);
+      }
+    }
+    return null;
+  }
+
+  parsePrefixOp(): Value | ErrorResult {
+    return (
+      this.tryAll(
+        Operators.filter((operator) => operator.leftBindingPower == -1)
+      ) ?? { error: "Couldn't parse number" }
+    );
+  }
+}
+
+// parse expression:
+// must parse sub-expression with right binding power 0
+// must be at EOF
+
+// parse sub-expression with right binding power :
+// must parse a prefix op
+// parse as many postfix ops w/ left binding power > right binding power
+
+// parse op:
+// must parse associated token (if success- failing the rest of the op fails *everything*)
+// depending on op:
+//   maybe parse sub-expressions with op's right-binding power
+//   maybe consume other tokens