no segfault yet

calc.c

@@ -0,0 +1,93 @@
+#include <stdio.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+
+typedef enum {
+    LITERAL,
+    ADD,
+    SUB,
+    MUL,
+    DIV,
+} ExprKind;
+
+struct Expr;
+
+typedef struct {
+    struct Expr *lhs;
+    struct Expr *rhs;
+} BinOp;
+
+typedef struct {
+    ExprKind tag;
+    union {
+        int64_t literal;
+        BinOp op;
+    } value;
+} Expr;
+
+bool is_numeric(const char character) {
+    return character >= '0' && character <= '9';
+}
+
+void parse_expr(char *input, size_t *offset) {
+}
+
+void parse_factor(char *input, size_t *offset) {
+}
+
+void parse_term(char *input, size_t *offset) {
+}
+
+Expr parse_literal(char *input, size_t *offset) {
+    // the index of the first character of the literal
+    const size_t start = *offset;
+
+    // the index of the character past the last character of the literal
+    size_t end = start;
+
+    while (input[*offset] != '\0' && is_numeric(input[*offset])) {
+        (*offset)++;
+        end = *offset;
+    }
+
+    size_t literal_len = end - start;
+
+    if (literal_len == 0) {
+        fprintf(stderr, "Found end of file expecting number");
+        exit(1);
+    }
+
+    // because C standard library functions are utterly deranged and want null terminated strings, we have to hack a little
+    // just set the next character to null to get a nice null terminated string
+    char old_next_char = input[literal_len];
+
+    int64_t value = strtoll(input + start, NULL, 10);
+
+    // and then don't forget to set it back :D
+    input[literal_len] = old_next_char;
+
+    Expr expr;
+    expr.tag = LITERAL;
+    expr.value.literal = value;
+
+    return expr;
+}
+
+int main(int argc, char **argv) {
+    if (argc == 1) {
+        printf("provide me an argument please\n");
+        return 1;
+    }
+
+    char *input = argv[1];
+
+    size_t offset = 0;
+
+    Expr expr = parse_literal(input, &offset);
+
+    printf("%ld\n", expr.value.literal);
+
+    return 0;
+}