Add aoc03.

03-joltage/.gitignore

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+aoc3
+*.o

03-joltage/Makefile

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+all: test
+
+.PHONY: test clean
+
+aoc3: aoc3.c
+	cc -O2 -o $@ $<
+
+clean:
+	-rm -f aoc3
+
+test: aoc3
+	./aoc3 < input01.txt

03-joltage/aoc3.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+int main(void)
+{
+	uint64_t total = 0;
+	uint8_t max = 0;
+	uint8_t prev = 0;
+	int8_t after = -1;
+
+	while (1) {
+		char c = getchar();
+
+		if (c >= '0' && c <= '9') {
+			uint8_t digit = c - '0';
+			if (digit > max) {
+				prev = max;
+				max = digit;
+				after = -1;
+			} else {
+				if (digit > after) {
+					after = digit;
+				}
+			}
+		} else if (c == '\n' || c == EOF) {
+			if (after == -1) {
+				// special case: max is the last digit
+				total += 10*prev + max;
+			} else {
+				total += 10*max + after;
+			}
+
+			max = 0;
+			prev = 0;
+			after = -1;
+
+			if (c == EOF) break;
+		} else {
+			fprintf(stderr, "weird character: %02x\n", c);
+			exit(1);
+		}
+	}
+
+	printf("%ld\n", total);
+	return 0;
+}