From c31d2b1918fd943f87e8288a608d73dc77f23d30 Mon Sep 17 00:00:00 2001
From: Micah Elizabeth Scott <micah@scanlime.org>
Date: Tue, 23 Jul 2013 17:43:32 -0700
Subject: [PATCH] Overflow-safe arithmetic in interpolation calculation
---
firmware/fadecandy.cpp | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/firmware/fadecandy.cpp b/firmware/fadecandy.cpp
index 53c30db..7bb4f5c 100644
--- a/firmware/fadecandy.cpp
+++ b/firmware/fadecandy.cpp
@@ -39,7 +39,7 @@ static OctoWS2811z leds(LEDS_PER_STRIP, ledBuffer, WS2811_800kHz);
static int8_t residual[CHANNELS_TOTAL];
-static uint32_t calculateInterpCoefficient()
+static inline uint32_t calculateInterpCoefficient()
{
/*
* Calculate our interpolation coefficient. This is a value between
@@ -62,9 +62,11 @@ static uint32_t calculateInterpCoefficient()
uint32_t now = millis();
uint32_t tsPrev = buffers.fbPrev->timestamp;
uint32_t tsNext = buffers.fbNext->timestamp;
+ uint32_t tsDiff = tsNext - tsPrev;
+ uint32_t tsElapsed = now - tsNext;
- uint32_t scaled = ((now - tsNext) << 16) / (tsNext - tsPrev);
- return std::min<uint32_t>(scaled, 0x10000);
+ // Careful to avoid overflows if the frames stop coming...
+ return (std::min<uint32_t>(tsElapsed, tsDiff) << 16) / tsDiff;
}
ALWAYS_INLINE static inline uint32_t lutInterpolate(const uint16_t *lut, uint32_t arg)
@@ -444,8 +446,11 @@ extern "C" int main()
leds.begin();
while (1) {
+ uint32_t ic = calculateInterpCoefficient();
+
buffers.handleUSB();
- updateDrawBuffer(calculateInterpCoefficient());
+
+ updateDrawBuffer(ic);
leds.show();
// Optionally disable dithering by clearing our residual buffer every frame.
--
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