Centre the digit field in the space it actually has

The field sat 13px above the centre of the band between its title and
its footer - measured, not eyeballed: the stack of arrows and boxes ran
26..77, centre 51, in a band of 18..110 whose centre is 64.

DIGIT_BOX_Y and the two arrow rows were pinned literals, so nothing
moved when the arrows changed height and nothing could notice they were
off-centre to begin with. They are derived from the stack's own parts
now, and from the band it sits in.

That band is not the same on both screens that draw this stack, which
is why one pinned y could never suit both: the digit field has only the
footer under it, while the value box shares its screen with the entropy
quality bar at DICE_BAR_Y. Each centres against the thing that actually
bounds it. The value box moves down 2px as a result - it was not centred
above the bar either, just less visibly.

Two things this turned up:

The assert meant to keep the down arrow off the footer compared against
LIST_FOOTER_Y, which belongs to the list screens and sits 11px below the
footer these actually draw. It would have permitted the overlap it was
written to catch. Both placements are asserted now, each against its own
bound.

The footer's y was the literal 111 in three renderers; it is
SCREEN_FOOTER_Y, which is what the centring has to measure against.

Verified with the host self-test and the 52 Python tests, and by
measuring the rendered framebuffer: the stack now centres at 63 against
the band's 64, the 1px being the odd height's rounding.

Author: oroderico

main/seedtool_render.c

@@ -48,6 +48,13 @@
 #define LIST_TOP 21
 #define LIST_ROW_HEIGHT 33
 #define LIST_FOOTER_Y 122
+
+/* The plain screens put their title at the top and their footer near the
+ * bottom; the band between is what the screen is for. Named because the digit
+ * field centres itself in it. */
+#define SCREEN_TITLE_Y 2
+#define SCREEN_TITLE_BOTTOM (SCREEN_TITLE_Y + 16) /* the 16px face's own height */
+#define SCREEN_FOOTER_Y 111
 #define LIST_GUTTER 10
 #define LIST_BAR_X 2
 #define LIST_BAR_WIDTH (SEEDTOOL_DISPLAY_WIDTH - LIST_GUTTER - 2 * LIST_BAR_X)
@@ -434,7 +441,7 @@ void seedtool_render_screen(const char* title, const char* line1, const char* li
      * small because 16px there would run off the bottom of the display. */
     draw_centered(tft_Ubuntu16, line1, 39);
     draw_centered(tft_Ubuntu16, line2, 65);
-    draw_centered(tft_DefaultFont, footer, 111);
+    draw_centered(tft_DefaultFont, footer, SCREEN_FOOTER_Y);
 }
 
 seedtool_thumb_t seedtool_list_thumb(const size_t count, const size_t top, const int track)
@@ -852,15 +859,23 @@ static void draw_progress_bar(const seedtool_progress_t* progress)
 #define DIGIT_BOX_WIDTH 34
 #define DIGIT_BOX_HEIGHT 32
 #define DIGIT_BOX_GAP 6
-#define DIGIT_BOX_Y 36
 #define DIGIT_ARROW_WIDTH 11
 /* Six rows, not seven: the half-width steps in whole pixels, so an 11-wide
  * triangle has exactly six distinct rows (1, 3, 5, 7, 9, 11). A seventh row
  * has no width left of its own and repeats one, which reads as a flat spot in
  * the edge rather than a straight diagonal. */
 #define DIGIT_ARROW_HEIGHT 6
-#define DIGIT_ARROW_UP_Y 26
-#define DIGIT_ARROW_DOWN_Y 71
+
+/* An arrow, the boxes, an arrow: one stack, sitting at a different height on
+ * each screen that draws it because what is under it differs - the value box
+ * shares its screen with the entropy quality bar, the digit field has only the
+ * footer. Derived from the stack's own parts, so changing an arrow's height
+ * moves the stack rather than silently unbalancing it. */
+#define DIGIT_FIELD_GAP 4
+#define DIGIT_FIELD_HEIGHT (2 * (DIGIT_ARROW_HEIGHT + DIGIT_FIELD_GAP) + DIGIT_BOX_HEIGHT)
+#define DIGIT_FIELD_TOP(bottom) (SCREEN_TITLE_BOTTOM + ((bottom) -SCREEN_TITLE_BOTTOM - DIGIT_FIELD_HEIGHT) / 2)
+#define DIGIT_FIELD_BOX_Y(top) ((top) + DIGIT_ARROW_HEIGHT + DIGIT_FIELD_GAP)
+#define DIGIT_FIELD_DOWN_Y(top) (DIGIT_FIELD_BOX_Y(top) + DIGIT_BOX_HEIGHT + DIGIT_FIELD_GAP)
 
 /* Held rather than eyeballed. The widest field is one box per digit a word
  * number can have, and it is centred, so a box or gap grown by a few pixels
@@ -871,9 +886,14 @@ static void draw_progress_bar(const seedtool_progress_t* progress)
 _Static_assert(SEEDTOOL_DIGIT_BOXES_MAX * DIGIT_BOX_WIDTH + (SEEDTOOL_DIGIT_BOXES_MAX - 1) * DIGIT_BOX_GAP
         <= SEEDTOOL_DISPLAY_WIDTH,
     "the digit boxes are wider than the display");
-_Static_assert(DIGIT_ARROW_UP_Y + DIGIT_ARROW_HEIGHT <= DIGIT_BOX_Y, "the up arrow runs into the digit boxes");
-_Static_assert(DIGIT_BOX_Y + DIGIT_BOX_HEIGHT <= DIGIT_ARROW_DOWN_Y, "the digit boxes run into the down arrow");
-_Static_assert(DIGIT_ARROW_DOWN_Y + DIGIT_ARROW_HEIGHT <= LIST_FOOTER_Y, "the down arrow runs into the footer");
+/* Both placements, each against the thing that actually bounds it. */
+_Static_assert(DIGIT_FIELD_TOP(SCREEN_FOOTER_Y) >= SCREEN_TITLE_BOTTOM, "the digit field runs into the title");
+_Static_assert(DIGIT_FIELD_DOWN_Y(DIGIT_FIELD_TOP(SCREEN_FOOTER_Y)) + DIGIT_ARROW_HEIGHT <= SCREEN_FOOTER_Y,
+    "the digit field runs into the footer");
+_Static_assert(DIGIT_FIELD_TOP(DICE_BAR_Y) >= SCREEN_TITLE_BOTTOM, "the value box runs into the title");
+_Static_assert(DIGIT_FIELD_DOWN_Y(DIGIT_FIELD_TOP(DICE_BAR_Y)) + DIGIT_ARROW_HEIGHT <= DICE_BAR_Y,
+    "the value box runs into the quality bar");
+
 
 /* One wider box for a value scrolled whole rather than digit by digit: a die
  * face is 1-6 and a D20 is 1-20, so splitting it into per-digit boxes would
@@ -885,21 +905,27 @@ void seedtool_render_value_box(
     const char* title, const char* text, const bool back, const char* footer, const seedtool_progress_t* progress)
 {
     seedtool_render_clear();
-    draw_centered(tft_Ubuntu16, title, 2);
+    draw_centered(tft_Ubuntu16, title, SCREEN_TITLE_Y);
+    /* Centred above the quality bar, not the footer: the bar is what shares
+     * this screen, whether or not `progress` draws one this time - the box
+     * should not jump between runs that grade their entropy and runs that do
+     * not. */
+    const int top = DIGIT_FIELD_TOP(DICE_BAR_Y);
+    const int box_y = DIGIT_FIELD_BOX_Y(top);
     const int x = (SEEDTOOL_DISPLAY_WIDTH - VALUE_BOX_WIDTH) / 2;
-    fill_rect(x, DIGIT_BOX_Y, VALUE_BOX_WIDTH, DIGIT_BOX_HEIGHT, COLOR_HIGHLIGHT);
-    draw_border(x, DIGIT_BOX_Y, VALUE_BOX_WIDTH, DIGIT_BOX_HEIGHT, COLOR_HIGHLIGHT);
+    fill_rect(x, box_y, VALUE_BOX_WIDTH, DIGIT_BOX_HEIGHT, COLOR_HIGHLIGHT);
+    draw_border(x, box_y, VALUE_BOX_WIDTH, DIGIT_BOX_HEIGHT, COLOR_HIGHLIGHT);
     if (back) {
         draw_backspace(x + (VALUE_BOX_WIDTH - BACKSPACE_WIDTH) / 2,
-            DIGIT_BOX_Y + (DIGIT_BOX_HEIGHT - BACKSPACE_HEIGHT) / 2, COLOR_BLACK, COLOR_HIGHLIGHT);
+            box_y + (DIGIT_BOX_HEIGHT - BACKSPACE_HEIGHT) / 2, COLOR_BLACK, COLOR_HIGHLIGHT);
     } else {
         draw_centered_in(tft_Ubuntu16, text, x, VALUE_BOX_WIDTH,
-            DIGIT_BOX_Y + (DIGIT_BOX_HEIGHT - tft_Ubuntu16[1]) / 2, COLOR_BLACK);
+            box_y + (DIGIT_BOX_HEIGHT - tft_Ubuntu16[1]) / 2, COLOR_BLACK);
     }
     const int arrow_x = x + (VALUE_BOX_WIDTH - DIGIT_ARROW_WIDTH) / 2;
-    draw_triangle(arrow_x, DIGIT_ARROW_UP_Y, DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_UP, COLOR_WHITE);
-    draw_triangle(arrow_x, DIGIT_ARROW_DOWN_Y, DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_DOWN, COLOR_WHITE);
-    draw_centered(tft_DefaultFont, footer, 111);
+    draw_triangle(arrow_x, top, DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_UP, COLOR_WHITE);
+    draw_triangle(arrow_x, DIGIT_FIELD_DOWN_Y(top), DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_DOWN, COLOR_WHITE);
+    draw_centered(tft_DefaultFont, footer, SCREEN_FOOTER_Y);
     if (progress) {
         draw_progress_bar(progress);
     }
@@ -909,29 +935,33 @@ void seedtool_render_digits(const char* title, const char* digits, const size_t
     const char* footer, const seedtool_progress_t* progress)
 {
     seedtool_render_clear();
-    draw_centered(tft_Ubuntu16, title, 2);
+    draw_centered(tft_Ubuntu16, title, SCREEN_TITLE_Y);
 
+    /* Nothing under this field but the footer, so it centres in that whole
+     * band rather than sharing the value box's higher placement. */
+    const int top = DIGIT_FIELD_TOP(SCREEN_FOOTER_Y);
+    const int box_y = DIGIT_FIELD_BOX_Y(top);
     const int span = (int)count * DIGIT_BOX_WIDTH + ((int)count - 1) * DIGIT_BOX_GAP;
     const int left = (SEEDTOOL_DISPLAY_WIDTH - span) / 2;
     for (size_t i = 0; i < count; ++i) {
         const int x = left + (int)i * (DIGIT_BOX_WIDTH + DIGIT_BOX_GAP);
         const bool selected = i == active;
         const bool set = i < active;
         if (selected) {
-            fill_rect(x, DIGIT_BOX_Y, DIGIT_BOX_WIDTH, DIGIT_BOX_HEIGHT, COLOR_HIGHLIGHT);
+            fill_rect(x, box_y, DIGIT_BOX_WIDTH, DIGIT_BOX_HEIGHT, COLOR_HIGHLIGHT);
         }
-        draw_border(x, DIGIT_BOX_Y, DIGIT_BOX_WIDTH, DIGIT_BOX_HEIGHT,
+        draw_border(x, box_y, DIGIT_BOX_WIDTH, DIGIT_BOX_HEIGHT,
             selected ? COLOR_HIGHLIGHT : set ? COLOR_WHITE : COLOR_DIM);
 
         const char shown = digits[i];
-        const int glyph_y = DIGIT_BOX_Y + (DIGIT_BOX_HEIGHT - tft_Ubuntu16[1]) / 2;
+        const int glyph_y = box_y + (DIGIT_BOX_HEIGHT - tft_Ubuntu16[1]) / 2;
         if (shown == SEEDTOOL_KEY_ACCEPT) {
             draw_centered_in(tft_Ubuntu16, "OK", x, DIGIT_BOX_WIDTH, glyph_y, selected ? COLOR_BLACK : COLOR_WHITE);
         } else if (shown == SEEDTOOL_KEY_BACKSPACE) {
             /* The way out of the field rides the same ring as the digits, so
              * the box shows the backspace glyph the keypad already uses for
              * it rather than a letter the reader has to decode. */
-            draw_backspace(x + (DIGIT_BOX_WIDTH - BACKSPACE_WIDTH) / 2, DIGIT_BOX_Y + (DIGIT_BOX_HEIGHT - BACKSPACE_HEIGHT) / 2,
+            draw_backspace(x + (DIGIT_BOX_WIDTH - BACKSPACE_WIDTH) / 2, box_y + (DIGIT_BOX_HEIGHT - BACKSPACE_HEIGHT) / 2,
                 COLOR_BLACK, COLOR_HIGHLIGHT);
         } else if (shown && shown != ' ') {
             const char text[2] = { shown, '\0' };
@@ -940,12 +970,13 @@ void seedtool_render_digits(const char* title, const char* digits, const size_t
 
         if (selected) {
             const int arrow_x = x + (DIGIT_BOX_WIDTH - DIGIT_ARROW_WIDTH) / 2;
-            draw_triangle(arrow_x, DIGIT_ARROW_UP_Y, DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_UP, COLOR_WHITE);
-            draw_triangle(arrow_x, DIGIT_ARROW_DOWN_Y, DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_DOWN, COLOR_WHITE);
+            draw_triangle(arrow_x, top, DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_UP, COLOR_WHITE);
+            draw_triangle(
+                arrow_x, DIGIT_FIELD_DOWN_Y(top), DIGIT_ARROW_WIDTH, DIGIT_ARROW_HEIGHT, TRIANGLE_DOWN, COLOR_WHITE);
         }
     }
 
-    draw_centered(tft_DefaultFont, footer, 111);
+    draw_centered(tft_DefaultFont, footer, SCREEN_FOOTER_Y);
     if (progress) {
         draw_progress_bar(progress);
     }