From d29d7b7a3dd2cfca151ce667a3290359d028911c Mon Sep 17 00:00:00 2001
From: Behdad Esfahbod <behdad@behdad.org>
Date: Wed, 1 Mar 2023 13:10:11 -0700
Subject: [PATCH] [algs] Adjust solve_itp

---
 src/hb-algs.hh  | 11 +++++++----
 src/hb-shape.cc |  4 ----
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/src/hb-algs.hh b/src/hb-algs.hh
index cf9403134..13587eac0 100644
--- a/src/hb-algs.hh
+++ b/src/hb-algs.hh
@@ -1344,7 +1344,10 @@ struct
 HB_FUNCOBJ (hb_dec);
 
 
-/* For documentation and implementation see:
+/* Adapted from kurbo implementation with extra parameters added,
+ * and finding for a particular range instead of 0.
+ *
+ * For documentation and implementation see:
  *
  * [ITP method]: https://en.wikipedia.org/wiki/ITP_Method
  * [An Enhancement of the Bisection Method Average Performance Preserving Minmax Optimality]: https://dl.acm.org/doi/10.1145/3423597
@@ -1355,12 +1358,12 @@ template <typename func_t>
 double solve_itp (func_t f,
 		  double a, double b,
 		  double epsilon,
-		  unsigned n0,
-		  double k1,
 		  double min_y, double max_y,
 		  double &ya, double &yb, double &y)
 {
   unsigned n1_2 = (unsigned) (hb_max (ceil (log2 ((b - a) / epsilon)) - 1.0, 0.0));
+  const unsigned n0 = 1; // Hardwired
+  const double k1 = 0.2 / (b - a); // Hardwired.
   unsigned nmax = n0 + n1_2;
   double scaled_epsilon = epsilon * double (1llu << nmax);
   double _2_epsilon = 2.0 * epsilon;
@@ -1370,8 +1373,8 @@ double solve_itp (func_t f,
     double r = scaled_epsilon - 0.5 * (b - a);
     double xf = (yb * a - ya * b) / (yb - ya);
     double sigma = x1_2 - xf;
-    // This has k2 = 2 hardwired for efficiency.
     double b_a = b - a;
+    // This has k2 = 2 hardwired for efficiency.
     double b_a_k2 = b_a * b_a;
     double delta = k1 * b_a_k2;
     int sigma_sign = sigma >= 0 ? +1 : -1;
diff --git a/src/hb-shape.cc b/src/hb-shape.cc
index 5aae008d7..3504288e4 100644
--- a/src/hb-shape.cc
+++ b/src/hb-shape.cc
@@ -334,8 +334,6 @@ hb_shape_justify (hb_font_t          *font,
   }
 
   double epsilon = (b - a) / (1<<14);
-  const unsigned n0 = 1;
-  const double k1 = 0.2 / (b - a);
   bool failed = false;
 
   auto f = [&] (double x)
@@ -361,8 +359,6 @@ hb_shape_justify (hb_font_t          *font,
   double itp = solve_itp (f,
 			  a, b,
 			  epsilon,
-			  n0,
-			  k1,
 			  min_target_width, max_target_width,
 			  ya, yb, y);