完成 part_3 的 exercise

Author: 汤杨

part_3/calc.ts

@@ -0,0 +1,117 @@
+// 用 ts 重新实现一遍 calc
+enum TokenType {
+    INTEGER,
+    PLUS,
+    MINUS
+}
+
+class Token {
+    public constructor(public token_type: TokenType, public value: string | number) {}
+}
+
+class Interpreter {
+    private text: string;
+    private current_token : Token;
+    private pos : number;
+    private current_char : string;
+    private number_reg: RegExp = new RegExp("^[0-9]$");
+
+    public constructor(text: string) {
+        this.text = text;
+        this.pos = 0;
+        this.current_char = this.text.charAt(this.pos);
+    }
+
+    private throwError(msg: string) {
+        throw new SyntaxError(msg);
+    }
+
+    private currentCharIsDigit() {
+        return this.number_reg.test(this.current_char);
+    }
+
+    private advance() {
+         this.pos += 1;
+        if (this.pos < this.text.length) {
+            this.current_char = this.text.charAt(this.pos);
+        } else {
+            this.current_char = null;
+        }
+    }
+
+    private getNumber() : number {
+        let start_pos = this.pos;
+        while (null != this.current_char && this.currentCharIsDigit()) {
+            this.advance();
+        }
+        let result = Number(this.text.substring(start_pos, this.pos));
+        return result;
+    }
+
+    private skipWhiteSpace() {
+        while (this.current_char != null && this.current_char == " ") {
+            this.advance();
+        }
+    }
+
+    private getNextToken() : Token {
+        while (this.current_char != null) {
+            if (this.current_char == " ") {
+                this.skipWhiteSpace();
+            } else if (this.currentCharIsDigit()) {
+                return new Token(TokenType.INTEGER, this.getNumber());
+            } else if (this.current_char == "+") {
+                this.advance();
+                return new Token(TokenType.PLUS, "+");
+            } else if (this.current_char == "-") {
+                this.advance();
+                return new Token(TokenType.MINUS, "-");
+            } else {
+                this.throwError("term should be number or plus or minus");
+            }
+        }
+        return null;
+    }
+
+    private assertTokenType(type: TokenType) {
+        if (this.current_token.token_type == type) {
+            this.current_token = this.getNextToken();
+            return;
+        }
+        this.throwError(`current token should be ${type}`);
+    }
+
+    private getTerm() : number {
+        let token = this.current_token;
+        this.assertTokenType(TokenType.INTEGER);
+        return token.value as number;
+    }
+
+    public interpreter() : number{
+        this.current_token = this.getNextToken();
+        let result = this.current_token.value as number;
+        this.assertTokenType(TokenType.INTEGER);
+
+        while (this.current_token != null && (this.current_token.token_type == TokenType.PLUS || this.current_token.token_type == TokenType.MINUS)) {
+            if (this.current_token.token_type == TokenType.PLUS) {
+                this.assertTokenType(TokenType.PLUS);
+                result += this.getTerm();
+            } else if (this.current_token.token_type == TokenType.MINUS) {
+                this.assertTokenType(TokenType.MINUS);
+                result -= this.getTerm();
+            } else {
+                this.throwError("Only support plus and minus");
+            }
+        }
+
+        return result;
+    }
+}
+
+const test_string_1: string = "1 + 2";
+const test_string_2: string = "1 + 2 + 3 - 5";
+const test_string_3: string = "12 + 32 - 3 + 321"
+
+console.log(`${test_string_1} = ${new Interpreter(test_string_1).interpreter()}`);
+console.log(`${test_string_2} = ${new Interpreter(test_string_2).interpreter()}`);
+console.log(`${test_string_3} = ${new Interpreter(test_string_3).interpreter()}`);

part_3/images/IMG_1096.JPG

@@ -1,12 +1,2 @@
 Q1: Draw a syntax diagram for arithmetic expressions that contain only multiplication and division, for example “7 * 4 / 2 * 3”. 
-```mermaid
-stateDiagram-v2
-[*] --> term1
-term1 --> nop
-nop --> *
-nop --> /
-* --> term2
-/ --> term2
-term2 --> nop 
-term2 --> [*]
-```
+![railroad diagram for multiplication and division](./images/IMG_1096.JPG)