From 4ff111b25a846cd5e93ee3c07bdb2e90190f6e7e Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Tue, 10 Nov 2020 18:22:26 +0100
Subject: [PATCH] add mandelbrot calculation

---
 meson.build      |   1 +
 src/mandelbrot.c | 130 +++++++++++++++++++++++++++++++++++++++++++++++
 src/mandelbrot.h |  10 ++++
 3 files changed, 141 insertions(+)
 create mode 100644 src/mandelbrot.c
 create mode 100644 src/mandelbrot.h

diff --git a/meson.build b/meson.build
index 23c07a8..a9a908d 100644
--- a/meson.build
+++ b/meson.build
@@ -11,6 +11,7 @@ project(
 sources = [
   'src/image.c',
   'src/main.c',
+  'src/mandelbrot.c',
   'src/options.c',
   'src/ppm.c',
 ]
diff --git a/src/mandelbrot.c b/src/mandelbrot.c
new file mode 100644
index 0000000..6a5c729
--- /dev/null
+++ b/src/mandelbrot.c
@@ -0,0 +1,130 @@
+#include "mandelbrot.h"
+
+#include <err.h>
+#include <stddef.h>
+#include <stdlib.h>
+
+#include "image.h"
+
+struct data {
+    float min_x;
+    float max_x;
+    float min_y;
+    float max_y;
+
+    size_t max_iter;
+
+    size_t h;
+    size_t w;
+
+    size_t buf[];
+};
+
+static size_t data_index(size_t h, size_t w, const struct data *d) {
+    return h * d->w + w;
+}
+
+static void normalize_histogram(float *h, size_t max_iter) {
+    float sum = 0;
+
+    // Don't add divergent points to the total
+    for (size_t i = 0; i < max_iter; ++i)
+        sum += h[i];
+
+    float running = 0;
+    for (size_t i = 0; i <= max_iter; ++i) {
+        running += h[i];
+        h[i] = running / sum;
+    }
+}
+
+static void compute_histogram(const struct data *d, float *h) {
+    // Histogram must be zero-initialized
+    for (size_t i = 0; i < d->h; ++i) {
+        for (size_t j = 0; j < d->w; ++j) {
+            size_t ind = data_index(i, j, d);
+            h[d->buf[ind]] += 1;
+        }
+    }
+
+    normalize_histogram(h, d->max_iter);
+}
+
+static float scale(float min, float max, size_t x, size_t width) {
+    return min + x * (max - min) / width;
+}
+
+static void compute_cycle(struct data *d, size_t h, size_t w) {
+    float x = scale(d->min_x, d->max_x, w, d->w);
+    float y = scale(d->min_y, d->max_y, h, d->h);
+
+
+    float i = 0.0;
+    float j = 0.0;
+    size_t k = 0;
+
+    while (i * i + j * j < 2 * 2 && k < d->max_iter) {
+        float xtemp = i * i - j * j + x;
+        j = 2 * i * j + y;
+        i = xtemp;
+        ++k;
+    }
+
+    d->buf[data_index(h, w, d)] =  k;
+}
+
+static void compute_mandelbrot(struct data *d) {
+    for (size_t i = 0; i < d->h; ++i) {
+        for (size_t j = 0; j < d->w; ++j) {
+            compute_cycle(d, i, j);
+        }
+    }
+}
+
+static void apply_lut(struct pixel *p, float x) {
+    const float x0 = 1.f / 4.f;
+    const float x1 = 2.f / 4.f;
+    const float x2 = 3.f / 4.f;
+
+    if (x < x0)
+        *p = (struct pixel){0, x / x0 * 255, 255};
+    else if (x < x1)
+        *p = (struct pixel){0, 255, (x1 - x) / x0 * 255};
+    else if (x < x2)
+        *p = (struct pixel){(x - x1) / x0 * 255, 255, 0};
+    else if (x < 1)
+        *p = (struct pixel){255, (1.f - x) / x0 * 255, 0};
+    else
+        *p = (struct pixel){0, 0, 0};
+
+}
+
+void mandelbrot(struct image *i, size_t max_iter) {
+    struct data *d = malloc(sizeof(*d->buf) * i->w * i->h + sizeof(*d));
+    if (!d)
+        err(EXIT_FAILURE, "could not allocate mandelbrot data");
+    float *h = calloc(sizeof(float), max_iter + 1);
+    if (!h) {
+        free(d);
+        err(EXIT_FAILURE, "could not allocate histogram");
+    }
+
+    d->h = i->h;
+    d->w = i->w;
+    d->max_iter = max_iter;
+
+    // FIXME" make it user-definable
+    d->min_x = -2.5;
+    d->max_x = 1.0;
+    d->min_y = -1.0;
+    d->max_y = 1.0;
+
+    compute_mandelbrot(d);
+    compute_histogram(d, h);
+
+    for (size_t ind = 0; ind < i->h * i->w; ++ind)
+        apply_lut(&i->buf[ind], h[d->buf[ind]]);
+
+    free(d);
+    free(h);
+}
diff --git a/src/mandelbrot.h b/src/mandelbrot.h
new file mode 100644
index 0000000..0b16b96
--- /dev/null
+++ b/src/mandelbrot.h
@@ -0,0 +1,10 @@
+#ifndef MANDELBROT_H
+#define MANDELBROT_H
+
+#include <stddef.h>
+
+struct image; // Forward declaration
+
+void mandelbrot(struct image *image, size_t max_iter);
+
+#endif /* !MANDELBROT_H */