diff --git a/day1/solution.c b/day1/solution.c
index bfed9b8..265a630 100644
--- a/day1/solution.c
+++ b/day1/solution.c
@@ -1,3 +1,5 @@
+#include <stdint.h>
+
 #include "parse.h"
 #include "heapsort.h"
 #include "abs.h"
@@ -23,7 +25,7 @@ static int parse_input(const char * input, int length, struct list * list)
   return i;
 }
 
-int day1_part1(const char * input, int length)
+int64_t day1_part1(const char * input, int length)
 {
   struct list list;
   int list_length = parse_input(input, length, &list);
@@ -40,7 +42,7 @@ int day1_part1(const char * input, int length)
   return sum;
 }
 
-int day1_part2(const char * input, int length)
+int64_t day1_part2(const char * input, int length)
 {
   struct list list;
   int list_length = parse_input(input, length, &list);
diff --git a/day2/solution.c b/day2/solution.c
index f2f2610..aa39937 100644
--- a/day2/solution.c
+++ b/day2/solution.c
@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdbool.h>
 
 #include "parse.h"
@@ -64,7 +65,7 @@ static bool report_safe(struct report * report, int skip)
   return true;
 }
 
-int day2_part1(const char * input, int length)
+int64_t day2_part1(const char * input, int length)
 {
   struct report report[1000];
 
@@ -78,7 +79,7 @@ int day2_part1(const char * input, int length)
   return sum;
 }
 
-int day2_part2(const char * input, int length)
+int64_t day2_part2(const char * input, int length)
 {
   struct report report[1000];
 
diff --git a/day3/solution.c b/day3/solution.c
index 20695f9..224b7ef 100644
--- a/day3/solution.c
+++ b/day3/solution.c
@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdbool.h>
 #include <stddef.h>
 
@@ -88,7 +89,7 @@ int parse_input(const char * input, int length, struct instruction * ins)
   return i;
 }
 
-int day3_part1(const char * input, int length)
+int64_t day3_part1(const char * input, int length)
 {
   struct instruction ins[1000];
 
@@ -103,7 +104,7 @@ int day3_part1(const char * input, int length)
   return sum;
 }
 
-int day3_part2(const char * input, int length)
+int64_t day3_part2(const char * input, int length)
 {
   struct instruction ins[1000];
 
diff --git a/day4/solution.c b/day4/solution.c
index c2c4f18..1d5724b 100644
--- a/day4/solution.c
+++ b/day4/solution.c
@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdbool.h>
 
 #include "printf.h"
@@ -131,7 +132,7 @@ static bool offset_match_cross(const char * input,
   return match1 || match2;
 }
 
-int day4_part1(const char * input, int length)
+int64_t day4_part1(const char * input, int length)
 {
   int stride = parse_stride(input, length);
   int height = parse_height(input, length);
@@ -159,7 +160,7 @@ int day4_part1(const char * input, int length)
   return match_count;
 }
 
-int day4_part2(const char * input, int length)
+int64_t day4_part2(const char * input, int length)
 {
   int stride = parse_stride(input, length);
   int height = parse_height(input, length);
diff --git a/day5/solution.c b/day5/solution.c
index 080fe36..03dfd02 100644
--- a/day5/solution.c
+++ b/day5/solution.c
@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdbool.h>
 
 #include "parse.h"
@@ -115,12 +116,12 @@ static int solve(const char * input, int length, bool correctly_ordered)
   return sum;
 }
 
-int day5_part1(const char * input, int length)
+int64_t day5_part1(const char * input, int length)
 {
   return solve(input, length, true);
 }
 
-int day5_part2(const char * input, int length)
+int64_t day5_part2(const char * input, int length)
 {
   return solve(input, length, false);
 }
diff --git a/day6/solution.c b/day6/solution.c
index b4732e1..4891821 100644
--- a/day6/solution.c
+++ b/day6/solution.c
@@ -1,5 +1,7 @@
+#include <stdint.h>
 #include <stdbool.h>
 #include <stddef.h>
+
 #include "printf.h"
 #include "parse.h"
 
@@ -128,7 +130,7 @@ void state_init(int part,
   day6_state.guard[0] = guard;
 }
 
-int day6_part1(const char * input, int length)
+int64_t day6_part1(const char * input, int length)
 {
   int stride = parse_stride(input, length);
   int height = parse_height(input, length);
@@ -203,7 +205,7 @@ static bool speculative_obstacle_causes_loop(const char * input,
   return false;
 }
 
-int day6_part2(const char * input, int length)
+int64_t day6_part2(const char * input, int length)
 {
   int stride = parse_stride(input, length);
   int height = parse_height(input, length);
diff --git a/day7/input.txt b/day7/input.txt
new file mode 100644
index 0000000..d2e9db3
--- /dev/null
+++ b/day7/input.txt
@@ -0,0 +1,850 @@
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diff --git a/day7/input.txt.h b/day7/input.txt.h
new file mode 100644
index 0000000..9c1d0e1
--- /dev/null
+++ b/day7/input.txt.h
@@ -0,0 +1,15 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern uint32_t _binary_day7_input_txt_start __asm("_binary_day7_input_txt_start");
+extern uint32_t _binary_day7_input_txt_end __asm("_binary_day7_input_txt_end");
+extern uint32_t _binary_day7_input_txt_size __asm("_binary_day7_input_txt_size");
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/day7/sample1.txt b/day7/sample1.txt
new file mode 100644
index 0000000..fc6e099
--- /dev/null
+++ b/day7/sample1.txt
@@ -0,0 +1,9 @@
+190: 10 19
+3267: 81 40 27
+83: 17 5
+156: 15 6
+7290: 6 8 6 15
+161011: 16 10 13
+192: 17 8 14
+21037: 9 7 18 13
+292: 11 6 16 20
diff --git a/day7/sample1.txt.h b/day7/sample1.txt.h
new file mode 100644
index 0000000..c79152d
--- /dev/null
+++ b/day7/sample1.txt.h
@@ -0,0 +1,15 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern uint32_t _binary_day7_sample1_txt_start __asm("_binary_day7_sample1_txt_start");
+extern uint32_t _binary_day7_sample1_txt_end __asm("_binary_day7_sample1_txt_end");
+extern uint32_t _binary_day7_sample1_txt_size __asm("_binary_day7_sample1_txt_size");
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/day7/solution.c b/day7/solution.c
new file mode 100644
index 0000000..e477a9c
--- /dev/null
+++ b/day7/solution.c
@@ -0,0 +1,121 @@
+#include <stdint.h>
+#include <stdbool.h>
+
+#include "parse.h"
+#include "printf.h"
+#include "unparse.h"
+
+struct equation {
+  int64_t value;
+  int operand[32];
+  int operand_count;
+};
+
+const char * parse_equation(const char * input, const char * end, struct equation * equation)
+{
+  input = parse_base10_64(input, &equation->value);
+  input = parse_skip(input, ':');
+  input = parse_skip(input, ' ');
+
+  equation->operand_count = 0;
+  while (*input != '\n') {
+    input = parse_base10(input, &equation->operand[equation->operand_count++]);
+    input = parse_skip(input, ' ');
+  }
+
+  return parse_skip(input, '\n');
+}
+
+static bool evaluate_equation1(struct equation * equation, int64_t acc, int depth)
+{
+  if (depth >= equation->operand_count)
+    return acc == equation->value;
+
+  int64_t operand = equation->operand[depth];
+
+  int64_t acc1 = acc * operand;
+  int64_t acc2 = acc + operand;
+  return
+       evaluate_equation1(equation, acc1, depth + 1)
+    || evaluate_equation1(equation, acc2, depth + 1);
+}
+
+int64_t day7_part1(const char * input, int length)
+{
+  const char * end = input + length;
+
+  int64_t sum = 0;
+  while (input < end) {
+    struct equation equation;
+    input = parse_equation(input, end, &equation);
+
+    bool possible = evaluate_equation1(&equation, equation.operand[0], 1);
+    if (possible)
+      sum += equation.value;
+  }
+
+  return sum;
+}
+
+static int64_t pow10[20] = {
+  1ull,
+  10ull,
+  100ull,
+  1000ull,
+  10000ull,
+  100000ull,
+  1000000ull,
+  10000000ull,
+  100000000ull,
+  1000000000ull,
+  10000000000ull,
+  100000000000ull,
+  1000000000000ull,
+  10000000000000ull,
+  100000000000000ull,
+  1000000000000000ull,
+  10000000000000000ull,
+  100000000000000000ull,
+  1000000000000000000ull,
+  10000000000000000000ull,
+};
+
+int64_t concatenate(int64_t a, int64_t b)
+{
+  int digits = digits_base10_64(b);
+  a *= pow10[digits];
+  return a + b;
+}
+
+static bool evaluate_equation2(struct equation * equation, int64_t acc, int depth)
+{
+  if (depth >= equation->operand_count)
+    return acc == equation->value;
+
+  int64_t operand = equation->operand[depth];
+
+  int64_t acc1 = acc * operand;
+  int64_t acc2 = acc + operand;
+  int64_t acc3 = concatenate(acc, operand);
+  return
+       evaluate_equation2(equation, acc1, depth + 1)
+    || evaluate_equation2(equation, acc2, depth + 1)
+    || evaluate_equation2(equation, acc3, depth + 1);
+}
+
+int64_t day7_part2(const char * input, int length)
+{
+  const char * end = input + length;
+
+  int64_t sum = 0;
+  while (input < end) {
+    struct equation equation;
+    input = parse_equation(input, end, &equation);
+
+    bool possible = evaluate_equation2(&equation, equation.operand[0], 1);
+    if (possible)
+      sum += equation.value;
+  }
+
+  return sum;
+}
diff --git a/gen.sh b/gen.sh
index 187bd36..72012df 100755
--- a/gen.sh
+++ b/gen.sh
@@ -13,6 +13,8 @@ if [ ! -z "$day" ]; then
 
     mkdir -p day${day}
     cat <<EOF > day${day}/solution.c
+#include <stdint.h>
+
 int day${day}_part1(const char * input, int length)
 {
   return -1;
@@ -96,8 +98,8 @@ echo >> solutions.mk
 
 truncate -s0 runner.inc
 for i in day* ; do
-    echo "int ${i}_part1(const char * input, int length);" >> runner.inc
-    echo "int ${i}_part2(const char * input, int length);" >> runner.inc
+    echo "int64_t ${i}_part1(const char * input, int length);" >> runner.inc
+    echo "int64_t ${i}_part2(const char * input, int length);" >> runner.inc
     if [ -f ${i}/render.cpp ]; then
         echo "void ${i}_render(const struct font * font," >> runner.inc
         echo "                 const struct glyph * glyphs," >> runner.inc
diff --git a/input_dreamcast.inc b/input_dreamcast.inc
index f870450..294768c 100644
--- a/input_dreamcast.inc
+++ b/input_dreamcast.inc
@@ -11,6 +11,8 @@
 #include "day5/input.txt.h"
 #include "day6/sample1.txt.h"
 #include "day6/input.txt.h"
+#include "day7/sample1.txt.h"
+#include "day7/input.txt.h"
 
 static struct start_size sample[][2] = {
   {
@@ -49,6 +51,12 @@ static struct start_size sample[][2] = {
     { (  char *)&_binary_day6_sample1_txt_start,
       (uint32_t)&_binary_day6_sample1_txt_size   },
   },
+  {
+    { (  char *)&_binary_day7_sample1_txt_start,
+      (uint32_t)&_binary_day7_sample1_txt_size   },
+    { (  char *)&_binary_day7_sample1_txt_start,
+      (uint32_t)&_binary_day7_sample1_txt_size   },
+  },
 };
 
 static struct start_size input[] = {
@@ -64,4 +72,6 @@ static struct start_size input[] = {
     (uint32_t)&_binary_day5_input_txt_size   },
   { (  char *)&_binary_day6_input_txt_start,
     (uint32_t)&_binary_day6_input_txt_size   },
+  { (  char *)&_binary_day7_input_txt_start,
+    (uint32_t)&_binary_day7_input_txt_size   },
 };
diff --git a/parse.c b/parse.c
index 516d092..87a0767 100644
--- a/parse.c
+++ b/parse.c
@@ -45,6 +45,23 @@ const char * parse_base10(const char * s, int * n)
   return s;
 }
 
+const char * parse_base10_64(const char * s, int64_t * n)
+{
+  *n = 0;
+
+  while (true) {
+    int digit = base10_digit(*s);
+    if (digit == -1)
+      break;
+
+    *n *= 10;
+    *n += digit;
+    s++;
+  }
+
+  return s;
+}
+
 const char * parse_match(const char * s, const char * m)
 {
   while (*m != 0) {
diff --git a/parse.h b/parse.h
index a246ea3..4eb5541 100644
--- a/parse.h
+++ b/parse.h
@@ -1,11 +1,14 @@
 #pragma once
 
+#include <stdint.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 const char * parse_skip(const char * s, char c);
 const char * parse_base10(const char * s, int * n);
+const char * parse_base10_64(const char * s, int64_t * n);
 const char * parse_match(const char * s, const char * m);
 int parse_stride(const char * s, int length);
 int parse_height(const char * s, int length);
diff --git a/printf.c b/printf.c
index f90dbd8..2f51539 100644
--- a/printf.c
+++ b/printf.c
@@ -8,6 +8,7 @@
 
 enum format_type {
   FORMAT_BASE10,
+  FORMAT_BASE10_64,
   FORMAT_BASE16,
   FORMAT_STRING,
   FORMAT_PERCENT,
@@ -38,6 +39,9 @@ static const char * parse_escape(const char * format, struct format * ft)
   case 'd':
     ft->type = FORMAT_BASE10;
     return format + 1;
+  case 'l':
+    ft->type = FORMAT_BASE10_64;
+    return format + 1;
   case 'x':
     ft->type = FORMAT_BASE16;
     return format + 1;
@@ -77,6 +81,14 @@ void _printf(const char * format, ...)
             global_output_buffer.buf_ix += offset;
           }
           break;
+        case FORMAT_BASE10_64:
+          {
+            int64_t num = va_arg(args, int64_t);
+            char * s = &global_output_buffer.buf[global_output_buffer.buf_ix];
+            int offset = unparse_base10_64(s, num, ft.pad_length, ft.fill_char);
+            global_output_buffer.buf_ix += offset;
+          }
+          break;
         case FORMAT_BASE16:
           {
             uint32_t num = va_arg(args, uint32_t);
diff --git a/runner.c b/runner.c
index 2a74b80..e81c807 100644
--- a/runner.c
+++ b/runner.c
@@ -1,11 +1,12 @@
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 
 #include "printf.h"
 #include "input.h"
 #include "runner.h"
 
-typedef int (* part_func)(const char * input, int length);
+typedef int64_t (* part_func)(const char * input, int length);
 typedef void (* render_func)(const struct font * font,
                              const struct glyph * glyphs,
                              const void * maple_ft0_data);
@@ -34,9 +35,9 @@ bool runner_tick(struct runner_state * runner_state)
   int length;
   //open_sample(day + 1, part + 1, &buf, &length);
   open_input(day + 1, &buf, &length);
-  int answer = solution[day].part[part](buf, length);
+  int64_t answer = solution[day].part[part](buf, length);
 
-  printf("day %d part %d: %d\n", day + 1, part + 1, answer);
+  printf("day %d part %d: %l\n", day + 1, part + 1, answer);
 
   runner_state->tick += 1;
 
diff --git a/runner.inc b/runner.inc
index 8a4707a..5968df7 100644
--- a/runner.inc
+++ b/runner.inc
@@ -1,18 +1,20 @@
-int day1_part1(const char * input, int length);
-int day1_part2(const char * input, int length);
-int day2_part1(const char * input, int length);
-int day2_part2(const char * input, int length);
-int day3_part1(const char * input, int length);
-int day3_part2(const char * input, int length);
-int day4_part1(const char * input, int length);
-int day4_part2(const char * input, int length);
-int day5_part1(const char * input, int length);
-int day5_part2(const char * input, int length);
-int day6_part1(const char * input, int length);
-int day6_part2(const char * input, int length);
+int64_t day1_part1(const char * input, int length);
+int64_t day1_part2(const char * input, int length);
+int64_t day2_part1(const char * input, int length);
+int64_t day2_part2(const char * input, int length);
+int64_t day3_part1(const char * input, int length);
+int64_t day3_part2(const char * input, int length);
+int64_t day4_part1(const char * input, int length);
+int64_t day4_part2(const char * input, int length);
+int64_t day5_part1(const char * input, int length);
+int64_t day5_part2(const char * input, int length);
+int64_t day6_part1(const char * input, int length);
+int64_t day6_part2(const char * input, int length);
 void day6_render(const struct font * font,
                  const struct glyph * glyphs,
                  const void * maple_ft0_data);
+int64_t day7_part1(const char * input, int length);
+int64_t day7_part2(const char * input, int length);
 
 struct day_funcs solution[] = {
   {
@@ -39,4 +41,8 @@ struct day_funcs solution[] = {
     {day6_part1, day6_part2},
     day6_render,
   },
+  {
+    {day7_part1, day7_part2},
+    NULL,
+  },
 };
diff --git a/solutions.mk b/solutions.mk
index 944a9e4..6023483 100644
--- a/solutions.mk
+++ b/solutions.mk
@@ -18,4 +18,7 @@ DAY_OBJ = \
 	day6/sample1.txt.o \
 	day6/input.txt.o \
 	day6/render.o \
-	day6/solution.o
+	day6/solution.o \
+	day7/sample1.txt.o \
+	day7/input.txt.o \
+	day7/solution.o
diff --git a/unparse.c b/unparse.c
index 5fd4ec7..f04eb94 100644
--- a/unparse.c
+++ b/unparse.c
@@ -2,18 +2,19 @@
 #include <stdbool.h>
 
 #include "minmax.h"
+#include "unparse.h"
 
-static int digits_base10(uint32_t n)
+int digits_base10(uint32_t n)
 {
-  if (n >= 1000000000) return 10;
-  if (n >= 100000000) return 9;
-  if (n >= 10000000) return 8;
-  if (n >= 1000000) return 7;
-  if (n >= 100000) return 6;
-  if (n >= 10000) return 5;
-  if (n >= 1000) return 4;
-  if (n >= 100) return 3;
-  if (n >= 10) return 2;
+  if (n >= 1000000000ul) return 10;
+  if (n >= 100000000ul) return 9;
+  if (n >= 10000000ul) return 8;
+  if (n >= 1000000ul) return 7;
+  if (n >= 100000ul) return 6;
+  if (n >= 10000ul) return 5;
+  if (n >= 1000ul) return 4;
+  if (n >= 100ul) return 3;
+  if (n >= 10ul) return 2;
   return 1;
 }
 
@@ -50,6 +51,63 @@ int unparse_base10(char * s, int32_t n, int len, char fill)
   return ret;
 }
 
+int digits_base10_64(uint64_t n)
+{
+  if (n >= 10000000000000000000ull) return 20;
+  if (n >= 1000000000000000000ull) return 19;
+  if (n >= 100000000000000000ull) return 18;
+  if (n >= 10000000000000000ull) return 17;
+  if (n >= 1000000000000000ull) return 16;
+  if (n >= 100000000000000ull) return 15;
+  if (n >= 10000000000000ull) return 14;
+  if (n >= 1000000000000ull) return 13;
+  if (n >= 100000000000ull) return 12;
+  if (n >= 10000000000ull) return 11;
+  if (n >= 1000000000ull) return 10;
+  if (n >= 100000000ull) return 9;
+  if (n >= 10000000ull) return 8;
+  if (n >= 1000000ull) return 7;
+  if (n >= 100000ull) return 6;
+  if (n >= 10000ull) return 5;
+  if (n >= 1000ull) return 4;
+  if (n >= 100ull) return 3;
+  if (n >= 10ull) return 2;
+  return 1;
+}
+
+int unparse_base10_64(char * s, int64_t n, int len, char fill)
+{
+  bool negative = false;
+  int digits = 0;
+  if (n < 0) {
+    digits += 1;
+    n = -n;
+    negative = true;
+  }
+
+  digits += digits_base10_64(n);
+  len = max(digits, len);
+  int ret = len;
+
+  while (len > digits) {
+    *s++ = fill;
+    --len;
+  }
+
+  if (negative) {
+    *s++ = '-';
+    len--;
+  }
+
+  while (len > 0) {
+    const uint32_t digit = n % 10;
+    n = n / 10;
+    s[--len] = digit + 48;
+  }
+
+  return ret;
+}
+
 static int digits_base16(uint32_t n)
 {
   if (n <= 0xf) return 1;
diff --git a/unparse.h b/unparse.h
index 2df3d49..fb588c9 100644
--- a/unparse.h
+++ b/unparse.h
@@ -4,9 +4,13 @@
 extern "C" {
 #endif
 
-int unparse_base10(char * s, uint32_t n, int len, char fill);
+int unparse_base10(char * s, int32_t n, int len, char fill);
+int unparse_base10_64(char * s, int64_t n, int len, char fill);
 int unparse_base16(char * s, uint32_t n, int len, char fill);
 
+int digits_base_64(uint64_t n);
+int digits_base10_64(uint64_t n);
+
 #ifdef __cplusplus
 }
 #endif