From d0a016cc3abd194efdeca077429230219a0ba881 Mon Sep 17 00:00:00 2001
From: Peter <peterr.00000@gmail.com>
Date: Tue, 7 Dec 2021 21:02:15 +0800
Subject: [PATCH] Day 7 solutions

---
 7/7-1.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 7/7-1.c

diff --git a/7/7-1.c b/7/7-1.c
new file mode 100644
index 0000000..a0005e0
--- /dev/null
+++ b/7/7-1.c
@@ -0,0 +1,84 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <math.h>
+#include <float.h>
+
+// SOLUTION USING GRADIENT DESCENT
+
+#define PART_2
+
+#ifdef PART_2
+#define STEP 0.0001F
+#else
+#define STEP 0.2F
+#endif
+
+#define INITIAL_CONDITION              \
+    {                                  \
+        16, 1, 2, 0, 4, 2, 7, 1, 2, 14 \
+    }
+#define INITIAL_GUESS 10000.0F
+
+// THIS IS NEEDED FOR PART 2 BECAUSE OF FLOATS. QUICK AND DIRTY COPYPASTE.
+int fuel_total_int(int *state, size_t state_len, int point)
+{
+    int total = 0;
+    for (size_t i = 0; i < state_len; i++)
+    {
+        int fuel = abs(state[i] - point);
+#ifdef PART_2
+        fuel *= (fuel + 1) / 2.0F; // TRIANGLE NUMBER T_n = n/2*(n+1)
+#endif
+        total += fuel;
+    }
+    return total;
+}
+
+// MUST BE FLOAT FOR GRADIENT DESCENT TO WORK.
+float fuel_total(int *state, size_t state_len, float point)
+{
+    float total = 0;
+    for (size_t i = 0; i < state_len; i++)
+    {
+        float fuel = fabs(state[i] - point);
+#ifdef PART_2
+        fuel *= (fuel + 1) / 2.0F; // TRIANGLE NUMBER T_n = n/2*(n+1)
+#endif
+        total += fuel;
+    }
+    return total;
+}
+
+void gradient_descent(int *state, size_t state_len, float *guess, float *gradient)
+{
+    float y = fuel_total(state, state_len, *guess);
+    float dy_dx = fuel_total(state, state_len, *guess + 1) - y;
+    printf("%.5f, y=%.5f, dy=%.5f\n", *guess, y, dy_dx);
+    *guess -= STEP * dy_dx;
+    *gradient = dy_dx;
+}
+
+float main(float argc, char const *argv[])
+{
+    int state[] = INITIAL_CONDITION;
+    size_t state_len = sizeof(state) / sizeof(state[0]);
+    float guess = INITIAL_GUESS;
+    float gradient = 1.0F; // START WITH NON-ZERO VALUE
+    while (fabs(gradient) > 0.01)
+    {
+        gradient_descent(state, state_len, &guess, &gradient);
+    }
+
+    // CHECK "AREA" SURROUNDING FINAL GUESS
+    int min_fuel = __INT_MAX__;
+    for (int t = guess - 1; t < guess + 1; t++)
+    {
+        int fuel = fuel_total_int(state, state_len, t);
+        if (fuel < min_fuel)
+            min_fuel = fuel;
+    }
+
+    printf("FUEL %d\n", (int)min_fuel);
+
+    return 0;
+}