From 6f09624280c394e189a45fd9f492147e3f5a189b Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Wed, 9 Oct 2019 12:45:19 +0530
Subject: [PATCH] When calculating CFI in paged mode go column-by-column from
 left to right and in each column start form midpoint and expand outwards

---
 src/pyj/read_book/paged_mode.pyj | 46 +++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 19 deletions(-)

diff --git a/src/pyj/read_book/paged_mode.pyj b/src/pyj/read_book/paged_mode.pyj
index 35f76c3197..b8a4584b43 100644
--- a/src/pyj/read_book/paged_mode.pyj
+++ b/src/pyj/read_book/paged_mode.pyj
@@ -401,25 +401,33 @@ def current_cfi():
     # null if it could not be calculated.
     ans = None
     if in_paged_mode():
-        c = current_column_location()
-        for x in c, c - col_and_gap, c + col_and_gap:
-            # Try the current column, the previous column and the next
-            # column. Each column is tried from top to bottom.
-            left, right = x, x + col_and_gap
-            if left < 0 or right > scroll_viewport.paged_content_width():
-                continue
-            deltax = col_and_gap // 25
-            deltay = scroll_viewport.height() // 25
-            cury = 0
-            while cury < scroll_viewport.height():
-                curx = left
-                while curx < right - gap:
-                    cfi = cfi_at_point(curx-Math.ceil(current_scroll_offset()), cury-Math.ceil(window.pageYOffset))
-                    if cfi:
-                        # print('Viewport cfi:', cfi)
-                        return cfi
-                    curx += deltax
-                cury += deltay
+        for cnum in range(cols_per_screen):
+            left = cnum * (col_and_gap + gap)
+            right = left + col_width
+            top, bottom = 0, scroll_viewport.height()
+            midx = (right - left) // 2
+            deltax = (right - left) // 24
+            deltay = (bottom - top) // 24
+            midy = (bottom - top) // 2
+            yidx = 0
+            while True:
+                yb, ya = midy - yidx * deltay, midy + yidx * deltay
+                if yb <= top or ya >= bottom:
+                    break
+                yidx += 1
+                xidx = 0
+                ys = v'[ya]' if ya is yb else v'[yb, ya]'
+                for cury in ys:
+                    xb, xa = midx - xidx * deltax, midx + xidx * deltax
+                    if xa <= left or xb >= right:
+                        break
+                    xidx += 1
+                    xs = v'[xa]' if xa is xb else v'[xb, xa]'
+                    for curx in xs:
+                        cfi = cfi_at_point(curx, cury)
+                        if cfi:
+                            # print('Viewport cfi:', cfi)
+                            return cfi
     else:
         try:
             ans = cfi_at_current() or None