From 271e1500774e8e5a05ea21ca9c79288da82f3870 Mon Sep 17 00:00:00 2001
From: bibin <bibin@soi.ch>
Date: Mon, 5 Jul 2021 15:24:31 +0200
Subject: [PATCH] add early version of binary search animation

---
 binary-search/binary-search-manim.py | 92 ++++++++++++++++++++++++++++
 binary-search/binary-search.py       |  7 +++
 2 files changed, 99 insertions(+)
 create mode 100644 binary-search/binary-search-manim.py
 create mode 100644 binary-search/binary-search.py

diff --git a/binary-search/binary-search-manim.py b/binary-search/binary-search-manim.py
new file mode 100644
index 0000000..8477117
--- /dev/null
+++ b/binary-search/binary-search-manim.py
@@ -0,0 +1,92 @@
+from manim import *
+
+
+class Sequence(VMobject):
+# evenly spaced characters
+
+    def __init__(self, seq):
+        VMobject.__init__(self)
+        seq = [t.move_to(i * 0.5 * RIGHT) for i, t in enumerate(seq)]
+        self.add(VGroup(*seq).move_to(ORIGIN + 2*UP))
+
+
+class Pointer(VMobject):
+# pointer to sequence element
+
+    def __init__(self, pointee, name):
+        VMobject.__init__(self)
+        arrow = Arrow(ORIGIN, DOWN).next_to(pointee, UP)
+        name_tex = Tex(name).next_to(arrow, UP)
+        self.add(arrow, name_tex)
+
+
+class ProgramCounter(VMobject):
+# pointer to current code line
+
+    def __init__(self, line):
+        VMobject.__init__(self)
+        self.add(Arrow(ORIGIN, RIGHT).next_to(line, LEFT))
+
+
+class Base(Scene):
+    def construct(self, n, k):
+
+        # initial situation
+        seq = [Tex("1")] + [Tex("?") for _ in range(n-2)] + [Tex("0")]
+        self.play(Create(Sequence(seq)))
+
+        # source code binary search
+        code = Code(file_name="binary-search.py", language="Python", insert_line_no=False, style='monokai')
+        code.move_to(DOWN)
+        self.play(Write(code))
+        line_lr, line_while, line_m, line_test, line_l, _, line_r = code[2]
+
+        def test(x):
+            return 1 if x <= k else 0
+
+        def update_pc(pc, line):
+            npc = ProgramCounter(line)
+            self.play(ReplacementTransform(pc, npc))
+            return npc
+
+        # initialize borders
+        pc = ProgramCounter(line_lr)
+        self.play(Create(pc))
+        l, r = 0, n-1
+        lp = Pointer(seq[l], "l")
+        rp = Pointer(seq[r], "r")
+        self.play(Write(lp), Create(rp))
+
+        while r - l > 1:
+            # calculate m
+            pc = update_pc(pc, line_m)
+            m = (l + r) // 2
+            mp = Pointer(seq[m], "m")
+            self.play(Create(mp))
+
+            # test
+            pc = update_pc(pc, line_test)
+            old = seq.copy()
+            seq[m] = Tex(str(test(m)))
+            self.play(ReplacementTransform(Sequence(old), Sequence(seq)))
+
+            if test(m):
+                # update left
+                pc = update_pc(pc, line_l)
+                l = m
+                op = lp
+                lp = Pointer(seq[m], "l")
+                self.play(Uncreate(mp), ReplacementTransform(op, lp))
+            else:
+                # update right
+                pc = update_pc(pc, line_r)
+                r = m
+                op = rp
+                rp = Pointer(seq[m], "r")
+                self.play(Uncreate(mp), ReplacementTransform(op, rp))
+
+        self.wait(2)
+
+class Main(Base):
+    def construct(self):
+        Base.construct(self, 20, 6)
diff --git a/binary-search/binary-search.py b/binary-search/binary-search.py
new file mode 100644
index 0000000..937361e
--- /dev/null
+++ b/binary-search/binary-search.py
@@ -0,0 +1,7 @@
+l, r = 0, n-1
+while r - l > 1:
+    m = (l + r) / 2
+    if test(m):
+        l = m
+    else:
+        r = m