1 files changed, 156 insertions, 0 deletions
diff --git a/roll.c b/roll.c
new file mode 100644
index 0000000..660eeab
--- /dev/null
+++ b/roll.c
@@ -0,0 +1,156 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+enum parser_state_e {
+    ST_NUMBER_BEFORE_DICE = 0,
+    ST_NUMBER_AFTER_DICE = 1,
+};
+
+struct state_s {
+    enum parser_state_e parser_state;
+    unsigned number_before_dice;
+    unsigned number_after_dice;
+    unsigned result;
+    int eof;
+    int syntax_error;
+};
+
+void print_usage(char* argv0)
+{
+    printf("usage: %s <command>\n", argv0);
+    puts("  examples: d20, 2d8+4, 1d6+d8+5");
+}
+
+unsigned diceroll(unsigned dice, unsigned rolls)
+{
+    unsigned result = 0;
+
+    for (; rolls > 0; rolls--) {
+        result += 1 + rand() % dice;
+    }
+
+    return result;
+}
+
+struct state_s eval_step(struct state_s state, char c)
+{
+    switch (c) {
+    case '0':
+    case '1':
+    case '2':
+    case '3':
+    case '4':
+    case '5':
+    case '6':
+    case '7':
+    case '8':
+    case '9':
+        /* anything numeric */
+        if (state.parser_state == ST_NUMBER_BEFORE_DICE) {
+            state.number_before_dice *= 10;
+            state.number_before_dice += c - '0';
+        } else if (state.parser_state == ST_NUMBER_AFTER_DICE) {
+            state.number_after_dice *= 10;
+            state.number_after_dice += c - '0';
+        }
+        break;
+    case 'd':
+    case 'D':
+        /* dice indicator */
+        if (state.parser_state == ST_NUMBER_BEFORE_DICE) {
+            if (state.number_before_dice == 0) {
+                /* allow shorthand, treat e.g. "d20" as "1d20" */
+                state.number_before_dice = 1;
+            }
+            state.parser_state = ST_NUMBER_AFTER_DICE;
+        } else {
+            /* not in the format ABdXY */
+            state.syntax_error = 1;
+        }
+        break;
+    case '+':
+    case '\0':
+        /* end of input or just end of an element */
+        if (state.number_after_dice > 0) {
+            state.result +=
+                diceroll(state.number_after_dice, state.number_before_dice);
+        } else if (state.parser_state == ST_NUMBER_BEFORE_DICE) {
+            /* we don't have a number after the dice */
+            /* in this case we just have a "naked" value that we take as-is */
+            state.result += state.number_before_dice;
+        } else {
+            /* missing number after dice indicator */
+            state.syntax_error = 1;
+            break;
+        }
+
+        state.number_before_dice = 0;
+        state.number_after_dice = 0;
+        state.parser_state = ST_NUMBER_BEFORE_DICE;
+
+        if (c == '\0') {
+            state.eof = 1;
+        }
+        break;
+    default:
+        state.syntax_error = 1;
+        break;
+    }
+
+    return state;
+}
+
+int eval(char* c)
+{
+    struct state_s state = {0};
+
+    while (1) {
+        state = eval_step(state, *c);
+
+        if (state.syntax_error) {
+            puts("syntax error");
+            return 1;
+        }
+        if (state.eof) {
+            break;
+        }
+
+        c++;
+    }
+
+    printf("%d\n", state.result);
+
+    return 0;
+}
+
+/**
+ * platform dependent way to generate a good seed for the random number
+ * generator
+ */
+unsigned get_seed()
+{
+    unsigned seed;
+
+#ifdef __linux__
+    FILE* fp = fopen("/dev/urandom", "rb");
+    fread(&seed, sizeof(unsigned), 1, fp);
+    fclose(fp);
+#else
+    seed = time(NULL);
+#endif
+
+    return seed;
+}
+
+int main(int argc, char* argv[])
+{
+    if (argc < 2) {
+        print_usage(argv[0]);
+        return 1;
+    }
+
+    srand(get_seed());
+
+    return eval(argv[1]);
+}