Draw axis if data crosses zero

[batteryviewer] / chart.c

/*
 * h/t https://ptomato.name/advanced-gtk-techniques/html/custom-container.html
 * for examples of how to make GTK widgets
 */

#include "chart.h"
#include <float.h>
#include <gtk/gtk.h>

#define BV_CHART_PRIVATE(obj)                                                  \
  (G_TYPE_INSTANCE_GET_PRIVATE((obj), BV_CHART_TYPE, BVChartPrivate))

typedef struct _BVChartPrivate BVChartPrivate;

struct _BVChartPrivate {
  GArray *points;
  float minx, miny, maxx, maxy;
};

struct BVChartPoint {
  float x, y;
};

struct ScreenPoint {
  float x, y;
};

G_DEFINE_TYPE_WITH_CODE(BVChart, bv_chart, GTK_TYPE_DRAWING_AREA,
                        G_ADD_PRIVATE(BVChart))

typedef void (*cairo_draw_func)(cairo_t *cr, double x, double y);

static struct ScreenPoint to_screen(BVChartPrivate *priv,
                                    GtkAllocation *allocation,
                                    struct BVChartPoint *p) {
  int margin = 3;
  float xscale = (allocation->width - 2 * margin) / (priv->maxx - priv->minx);
  float yscale = (allocation->height - 2 * margin) / (priv->miny - priv->maxy);
  struct ScreenPoint screen_p = {
      .x = xscale * (p->x - priv->minx) + margin,
      .y = yscale * (p->y - priv->maxy) + margin,
  };
  return screen_p;
}

static void draw_data(BVChartPrivate *priv, cairo_t *cr,
                      GtkAllocation *allocation) {
  cairo_set_source_rgb(cr, 0.0, 0.0, 1.0);
  float gap_threshold = 3.0;
  float prevx = 0.0;
  for (guint i = 0; i < priv->points->len; i++) {
    struct BVChartPoint *p =
        &g_array_index(priv->points, struct BVChartPoint, i);
    gboolean is_gap = p->x - prevx > gap_threshold;
    cairo_draw_func f = is_gap ? cairo_move_to : cairo_line_to;
    struct ScreenPoint screen_p = to_screen(priv, allocation, p);
    f(cr, screen_p.x, screen_p.y);
    prevx = p->x;
  }
  cairo_stroke(cr);
}

static void draw_axis(BVChartPrivate *priv, cairo_t *cr,
                      GtkAllocation *allocation) {
  if (priv->miny < 0.0 && priv->maxy > 0.0) {
    cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
    struct BVChartPoint p = {.x = priv->minx, .y = 0.0};
    struct ScreenPoint screen_p = to_screen(priv, allocation, &p);
    cairo_move_to(cr, screen_p.x, screen_p.y);
    p.x = priv->maxx;
    screen_p = to_screen(priv, allocation, &p);
    cairo_line_to(cr, screen_p.x, screen_p.y);
    cairo_stroke(cr);
  }
}

static gboolean bv_chart_draw(GtkWidget *widget, cairo_t *cr) {
  BVChart *chart = BV_CHART(widget);
  BVChartPrivate *priv = bv_chart_get_instance_private(chart);

  cairo_set_source_rgb(cr, 1.0, 1.0, 1.0);
  cairo_paint(cr);

  if (priv->points->len < 2)
    return TRUE;

  GtkAllocation allocation;
  gtk_widget_get_allocation(widget, &allocation);

  draw_axis(priv, cr, &allocation);
  draw_data(priv, cr, &allocation);

  return TRUE;
}

static void bv_chart_class_init(BVChartClass *klass) {
  GtkWidgetClass *widget_class = GTK_WIDGET_CLASS(klass);
  widget_class->draw = bv_chart_draw;
}

static void bv_chart_init(BVChart *chart) {
  gtk_widget_set_has_window(GTK_WIDGET(chart), FALSE);
  BVChartPrivate *priv = bv_chart_get_instance_private(chart);
  gboolean zero_terminated = FALSE;
  gboolean clear_ = FALSE;
  priv->points =
      g_array_new(zero_terminated, clear_, sizeof(struct BVChartPoint));
  priv->minx = FLT_MAX;
  priv->miny = FLT_MAX;
  priv->maxx = FLT_MIN;
  priv->maxy = FLT_MIN;
}

GtkWidget *bv_chart_new() {
  return GTK_WIDGET(g_object_new(bv_chart_get_type(), NULL));
}

void bv_chart_add_point(BVChart *chart, float x, float y) {
  BVChartPrivate *priv = bv_chart_get_instance_private(chart);
  if (x < priv->minx)
    priv->minx = x;
  if (y < priv->miny)
    priv->miny = y;
  if (x > priv->maxx)
    priv->maxx = x;
  if (y > priv->maxy)
    priv->maxy = y;
  struct BVChartPoint p = {.x = x, .y = y};
  g_array_append_val(priv->points, p);
}