From af8e7eae8e426a1f6c0bd29f6640424bf613bdf0 Mon Sep 17 00:00:00 2001
From: Hugo <hajenzoo29@gmail.com>
Date: Thu, 28 Jun 2018 10:15:27 +0200
Subject: [PATCH 1/2] Add Python 3 code to Verlet

---
 CONTRIBUTORS.md                               |  1 +
 .../verlet/code/python3/verlet.py             | 67 +++++++++++++++++++
 chapters/physics_solvers/verlet/verlet.md     |  9 +++
 3 files changed, 77 insertions(+)
 create mode 100644 chapters/physics_solvers/verlet/code/python3/verlet.py

diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index a1ad4161d..a5e2e11b7 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -5,3 +5,4 @@ Gathros
 Jeremie Gillet (- Jie -)
 Salim Khatib
 Hitesh C
+Hugo Granström
diff --git a/chapters/physics_solvers/verlet/code/python3/verlet.py b/chapters/physics_solvers/verlet/code/python3/verlet.py
new file mode 100644
index 000000000..7882e93e3
--- /dev/null
+++ b/chapters/physics_solvers/verlet/code/python3/verlet.py
@@ -0,0 +1,67 @@
+# submitted by HugoGranstrom
+
+def verlet_step(pos_prev, pos, acc, dt):
+    pos_next = 2 * pos - pos_prev + acc * dt * dt
+    return pos_next
+
+# calculate velocity at the current timestep
+def stormer_verlet_current(pos_prev, pos_next, dt):
+    vel_current = (pos_next - pos_prev) / (2 * dt)
+    return vel_current
+
+#calculate velocity at the next timestep
+def stormer_verlet_next(pos_next, pos, dt):
+    vel_next = (pos_next - pos) / (dt)
+    return vel_next
+
+def acc_func(pos):
+    # write the function for acceleration here, for example acc = F/m 
+    # now acc is constant and independent of pos, thus pos is not used
+    # for other implentations it can be used though
+    return -10
+
+def velocity_verlet_step(pos, vel, acc, dt):
+    pos_next = pos + vel * dt + 0.5 * acc * dt * dt
+    acc_next = acc_func(pos_next)
+    vel_next = vel + 0.5 * (acc + acc_next) * dt
+    return pos_next, vel_next, acc_next
+
+# HugoGranstrom
+# example calculating time it takes for an object to fall 5 m with acc = -10
+# because acc is constant, all 0.5 * (acc + acc_next) * dt becomes just acc * dt
+
+def run_verlet(pos_start, acc, dt):
+    time = 0
+    pos = pos_start
+    pos_prev = pos_start
+
+    while pos > 0:
+        # calculate next timestep
+        pos_next = 2 * pos - pos_prev + acc * dt * dt
+
+        # prepare for next timestep
+        pos_prev = pos
+        pos = pos_next
+        time += dt
+    
+    print(f"Classic Verlet took {time} seconds")
+
+def run_velocity_verlet(pos_start, vel_start, acc, dt):
+    time = 0
+    pos = pos_start
+    vel = vel_start
+
+    while pos > 0:
+        # calculate next timestep
+        pos_next = pos + vel * dt + 0.5 * acc * dt * dt
+        vel_next = vel + acc * dt
+
+        # prepare for next timestep
+        pos = pos_next
+        vel = vel_next
+        time += dt
+    
+    print(f"Velocity Verlet took {time} seconds")
+
+run_verlet(5, -10, 0.01)
+run_velocity_verlet(5, 0, -10, 0.01)
\ No newline at end of file
diff --git a/chapters/physics_solvers/verlet/verlet.md b/chapters/physics_solvers/verlet/verlet.md
index b936601a5..4760f144a 100644
--- a/chapters/physics_solvers/verlet/verlet.md
+++ b/chapters/physics_solvers/verlet/verlet.md
@@ -40,6 +40,8 @@ Here is what it looks like in code:
 [import:2-18, lang:"java"](code/java/verlet.java)
 {% sample lang="py2" %}
 [import:28-33, lang:"python"](code/python2/verlet.py)
+{% sample lang="py3" %}
+[import:3-5, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 Unfortunately, this has not yet been implemented in haskell, so here's Julia code:
 [import:1-13, lang:"julia"](code/julia/verlet.jl)
@@ -86,6 +88,8 @@ Here's what it looks like in code:
 [import:21-40, lang:"java"](code/java/verlet.java)
 {% sample lang="py2" %}
 [import:35-42, lang:"python"](code/python2/verlet.py)
+{% sample lang="py3" %}
+[import:7-15, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 Unfortunately, this has not yet been implemented in scratch, so here's Julia code:
 [import:15-31, lang:"julia"](code/julia/verlet.jl)
@@ -143,6 +147,8 @@ Here is the velocity Verlet method in code:
 [import:43-57, lang:"java"](code/java/verlet.java)
 {% sample lang="py2" %}
 [import:44-48, lang:"python"](code/python2/verlet.py)
+{% sample lang="py3" %}
+[import:17-27, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 Unfortunately, this has not yet been implemented in haskell, so here's Julia code:
 [import:33-45, lang:"julia"](code/julia/verlet.jl)
@@ -184,6 +190,9 @@ Both of these methods work simply by iterating timestep-by-timestep and can be w
 {% sample lang="py2" %}
 ### Python
 [import, lang:"python"](code/python2/verlet.py)
+{% sample lang="py3" %}
+### Python 3
+[import:29-67, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 ### Haskell
 [import, lang:"haskell"](code/haskell/verlet.hs)

From 70b73317a2a5d6242aaa51b6bb9347d91bada0aa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20Granstr=C3=B6m?=
 <5092565+HugoGranstrom@users.noreply.github.com>
Date: Sat, 30 Jun 2018 17:53:01 +0200
Subject: [PATCH 2/2] Change to "py3" to "py"

---
 chapters/physics_solvers/verlet/verlet.md | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/chapters/physics_solvers/verlet/verlet.md b/chapters/physics_solvers/verlet/verlet.md
index 4760f144a..8cf328672 100644
--- a/chapters/physics_solvers/verlet/verlet.md
+++ b/chapters/physics_solvers/verlet/verlet.md
@@ -40,7 +40,7 @@ Here is what it looks like in code:
 [import:2-18, lang:"java"](code/java/verlet.java)
 {% sample lang="py2" %}
 [import:28-33, lang:"python"](code/python2/verlet.py)
-{% sample lang="py3" %}
+{% sample lang="py" %}
 [import:3-5, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 Unfortunately, this has not yet been implemented in haskell, so here's Julia code:
@@ -88,7 +88,7 @@ Here's what it looks like in code:
 [import:21-40, lang:"java"](code/java/verlet.java)
 {% sample lang="py2" %}
 [import:35-42, lang:"python"](code/python2/verlet.py)
-{% sample lang="py3" %}
+{% sample lang="py" %}
 [import:7-15, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 Unfortunately, this has not yet been implemented in scratch, so here's Julia code:
@@ -147,7 +147,7 @@ Here is the velocity Verlet method in code:
 [import:43-57, lang:"java"](code/java/verlet.java)
 {% sample lang="py2" %}
 [import:44-48, lang:"python"](code/python2/verlet.py)
-{% sample lang="py3" %}
+{% sample lang="py" %}
 [import:17-27, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 Unfortunately, this has not yet been implemented in haskell, so here's Julia code:
@@ -190,8 +190,8 @@ Both of these methods work simply by iterating timestep-by-timestep and can be w
 {% sample lang="py2" %}
 ### Python
 [import, lang:"python"](code/python2/verlet.py)
-{% sample lang="py3" %}
-### Python 3
+{% sample lang="py" %}
+### Python
 [import:29-67, lang="python"](code/python3/verlet.py)
 {% sample lang="hs" %}
 ### Haskell