From 369fb56365c98855f31f57a05f425ab54c7fa603 Mon Sep 17 00:00:00 2001
From: Pete Bristow <pete.bristow@gmail.com>
Date: Wed, 30 Mar 2016 19:55:26 +0100
Subject: [PATCH 1/2] basic truncate / sample apps

---
 src/apps/basic/README.src.md  | 22 +++++++++++++++++++++
 src/apps/basic/basic_apps.lua | 37 +++++++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/src/apps/basic/README.src.md b/src/apps/basic/README.src.md
index 7473b17abf..b38c5c9c74 100644
--- a/src/apps/basic/README.src.md
+++ b/src/apps/basic/README.src.md
@@ -37,6 +37,17 @@ from the inputs onto the output.
               |        |
               +--------+
 
+## Sample
+The `Sample` app forwards packets every `n`th packet from `input`
+to `output` all others are dropped.
+
+    DIAGRAM: Sample
+              +--------+
+              |        |
+    input ----* Sample *---- output
+              |        |
+              +--------+
+
 ## Split
 
 The `Split` app splits packets from multiple inputs across multiple
@@ -87,6 +98,17 @@ merge and/or duplicate packet streams
               |        |
               +--------+
 
+## Truncate
+The `Truncate` app sends all packets received on `input` to `output`
+and truncates or zero pads packets to length `n` in the process
+
+    DIAGRAM: Sample
+              +----------+
+              |          |
+    input ----* Truncate *---- output
+              |          |
+              +----------+
+
 ## Repeater
 
 The `Repeater` app collects all packets received from the `input` link
diff --git a/src/apps/basic/basic_apps.lua b/src/apps/basic/basic_apps.lua
index ca2cb35093..b966fd3e7f 100644
--- a/src/apps/basic/basic_apps.lua
+++ b/src/apps/basic/basic_apps.lua
@@ -12,6 +12,43 @@ local transmit, receive = link.transmit, link.receive
 local ffi = require("ffi")
 local C = ffi.C
 
+--- # `Truncate` app: truncate or zero pad packet to length n
+Truncate = {}
+function Truncate:new(n)
+   return setmetatable({n = n}, {__index=Truncate})
+end
+function Truncate:push()
+   for _ = 1, link.nreadable(self.input.input) do
+      local p = receive(self.input.input)
+      if p.length > self.n then
+         p.length = self.n
+      else
+         for i = self.n - p.length, self.n do
+            p.data[i] = 0
+         end
+      end
+      transmit(self.output.output,p)
+   end
+end
+
+--- # `Sample` app: let through every nth packet
+Sample = {}
+function Sample:new(n)
+   return setmetatable({n = n, seen = 1}, {__index=Sample})
+end
+function Sample:push()
+   for _ = 1, link.nreadable(self.input.input) do
+      local p = receive(self.input.input)
+      if self.n == self.seen then
+         transmit(self.output.output, p)
+         self.seen = 1
+      else
+         self.seen = self.seen + 1
+         packet.free(p)
+      end
+   end
+end
+
 --- # `Source` app: generate synthetic packets
 
 Source = {}

From 41beb994b361cc0c345f0671b0f5fcd7a9c270af Mon Sep 17 00:00:00 2001
From: Pete Bristow <pete.bristow@gmail.com>
Date: Tue, 26 Apr 2016 23:52:20 +0100
Subject: [PATCH 2/2] use ffi.fill

---
 src/apps/basic/basic_apps.lua | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/src/apps/basic/basic_apps.lua b/src/apps/basic/basic_apps.lua
index b966fd3e7f..e92aae988f 100644
--- a/src/apps/basic/basic_apps.lua
+++ b/src/apps/basic/basic_apps.lua
@@ -6,11 +6,8 @@ local app = require("core.app")
 local freelist = require("core.freelist")
 local packet = require("core.packet")
 local link = require("core.link")
-local transmit, receive = link.transmit, link.receive
-
-
 local ffi = require("ffi")
-local C = ffi.C
+local transmit, receive = link.transmit, link.receive
 
 --- # `Truncate` app: truncate or zero pad packet to length n
 Truncate = {}
@@ -20,13 +17,8 @@ end
 function Truncate:push()
    for _ = 1, link.nreadable(self.input.input) do
       local p = receive(self.input.input)
-      if p.length > self.n then
-         p.length = self.n
-      else
-         for i = self.n - p.length, self.n do
-            p.data[i] = 0
-         end
-      end
+      ffi.fill(p.data, math.min(0, self.n - p.length))
+      p.length = self.n
       transmit(self.output.output,p)
    end
 end