Calculator, lexer and parser

calc.c

@@ -0,0 +1,129 @@
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef enum {
+    TOKEN_INT,
+    TOKEN_PLUS,
+    TOKEN_MUL,
+    TOKEN_DIV,
+    TOKEN_LPAREN,
+    TOKEN_RPAREN,
+    TOKEN_EOF
+} TokenType;
+
+typedef struct {
+    TokenType type;
+    int value;
+} Token;
+
+const char *input;
+size_t pos;
+Token current_token;
+
+void skip_whitespace() {
+    while (isspace(input[pos])) pos++;
+}
+
+Token get_next_token() {
+    skip_whitespace();
+
+    char ch = input[pos];
+
+    if (ch == '\0')
+        return (Token){TOKEN_EOF, 0};
+
+    if (isdigit(ch)) {
+        int val = 0;
+        while (isdigit(input[pos])) {
+            val = val * 10 + (input[pos++] - '0');
+        }
+        return (Token){TOKEN_INT, val};
+    }
+
+    pos++;
+    switch (ch) {
+    case '+':
+        return (Token){TOKEN_PLUS, 0};
+    case '*':
+        return (Token){TOKEN_MUL, 0};
+    case '/':
+        return (Token){TOKEN_DIV, 0};
+    case '(':
+        return (Token){TOKEN_LPAREN, 0};
+    case ')':
+        return (Token){TOKEN_RPAREN, 0};
+    default:
+        fprintf(stderr, "Unexpected character: %c\n", ch);
+        exit(1);
+    }
+}
+
+void eat(TokenType type) {
+    if (current_token.type == type) {
+        current_token = get_next_token();
+    } else {
+        fprintf(stderr, "Unexpected token\n");
+        exit(1);
+    }
+}
+
+int expr();
+
+int factor() {
+    Token tok = current_token;
+
+    if (tok.type == TOKEN_INT) {
+        eat(TOKEN_INT);
+        return tok.value;
+    } else if (tok.type == TOKEN_LPAREN) {
+        eat(TOKEN_LPAREN);
+        int result = expr();
+        eat(TOKEN_RPAREN);
+        return result;
+    } else {
+        fprintf(stderr, "Expected integer or '('\n");
+        exit(1);
+    }
+}
+
+int term() {
+    int result = factor();
+    while (current_token.type == TOKEN_MUL) {
+        eat(TOKEN_MUL);
+        result *= factor();
+    }
+    while (current_token.type == TOKEN_DIV) {
+        eat(TOKEN_DIV);
+        result /= factor();
+    }
+    return result;
+}
+
+int expr() {
+    int result = term();
+    while (current_token.type == TOKEN_PLUS) {
+        eat(TOKEN_PLUS);
+        result += term();
+    }
+    return result;
+}
+
+int main() {
+    char buffer[256];
+    printf("Enter expression: ");
+    fgets(buffer, sizeof(buffer), stdin);
+
+    input = buffer;
+    pos = 0;
+    current_token = get_next_token();
+
+    int result = expr();
+    if (current_token.type != TOKEN_EOF) {
+        fprintf(stderr, "Unexpected extra input\n");
+        return 1;
+    }
+
+    printf("Result = %d\n", result);
+    return 0;
+}