Jeff Epler's blog

Code size for bit reversing algorithms

Code space is always at a premium, so I investigated several code fragments for bit reversal to find out which was smallest on arm and xtensa. These code fragments are gathered from the internet. The loop is the smallest alternative on both architectures, but the 16-element look up table is not much bigger (on arm, the difference is bigger on xtensa) and is probably faster.

arm-none-eabi-gcc-7.2.1 -Os -mthumb

   text    data     bss     dec     hex filename
     40       0       0      40      28 bitrev_loop.o
     44       0       0      44      2c bitrev_lut16.o
     44       0       0      44      2c bitrev_shifts1.o
     44       0       0      44      2c bitrev_twiddle1.o

xtensa-lx106-elf-gcc-4.8.5 -Os

   text    data     bss     dec     hex filename
     40       0       0      40      28 bitrev_loop.o
     53       0       0      53      35 bitrev_lut16.o
     57       0       0      57      39 bitrev_shifts1.o
     52       0       0      52      34 bitrev_twiddle1.o

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Fuzz-testing CircuitPython

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afl-graphs

I recommend that you use a throwaway virtual machine for this, because at one point afl-fuzz learned how to create files in the filesystem! that was a big surprise, waking up to a directory full of filenames like "tesppppppppppppppppppppppppppppptfile"!

First, make sure you can build circuitpython's unix port. The steps are, approximately,

1. Clone circuitpython
2. git submodule update --init --recursive
3. make -C ports/unix -j5 deplibs
4. make -C ports/unix -j5

Note that the executable is ports/unix/micropython even when you have cloned circuitpython.

Next, get afl from http://lcamtuf.coredump.cx/afl/. If you can, follow the instructions in llvm_mode/README.llvm to get afl-clang-fast. Now, clean and rebuild:

1. make -C ports/unix clean
2. make -C ports/unix CC=/path/to/afl-clang-fast -j5 deplibs
3. make -C ports/unix CC=/path/to/afl-clang-fast -j5

Prepare the testcases directory for afl-fuzz. I used a number of tests from tests/basic:

1. mkdir testcases
2. cp tests/basics/*.py testcases

And start the fuzzer:

1. /path/to/afl-fuzz -i testcases -o findings -- ports/unix/circuitpython

If you have any good findings, drop by the adafruit circuitpython discord and let us know about them! Even better if you fix them.

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Watch Repairs

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My Grandfather's Hamilton Railroad Watch, repaired by Chris Radek

After a 15-year break, I'm now accepting repair work again!

I do everything from routine cleaning and oiling (full disassembly and inspection, multi-stage ultrasonic cleaning, reassembly and correct oiling using appropriate modern synthetic oils, timing and adjustment as necessary) of automatic and manual-wind wrist and pocket watches to manufacture of unavailable parts for chronographs or repeaters.

I specialize in Accutron tuning fork watches and I have a large stock of parts and many years of experience working on them.

Please contact me and tell me how I can help you. If you are a collector I've previously worked for, I'd especially like to hear from you.

His work winding new coils for Accutron watches is particularly exciting; he tells me he's heard of just one other person doing this in the last 20 years. These particular watch parts are otherwise irreplacable.

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Finding all numbers produced by short, digitless 'GNU dc' programs

A few years ago, possibly in 2014, I saw this code golf problem, which invites you write a short program that prints "2014"—the caveat being, the program may not contain any digits in its source code.

read more…

Four years of Prius (2013) fuel efficiency

Fuel economy and total mileage both went up in 2017 compared to earlier years. However, due to taking more road trips total fuel usage went up.

This year had my best mileage on a single nontrivial trip, shown as 99.9 MPG on 58.5 miles. This drive included significant downhill portions (descending from 12,000 feet to 6000 feet, if I recall correctly).

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poc: Python OCE Composer

Here you can see that the input format is fairly terse, and it has some abilities OpenSCAD doesn't, like selectively filleting certain edges:

with Difference():
    Box((-50,-50,-50), (50,50,50))
    Cylinder((-100,0,0), (100,0,0), 25)
    Cylinder((0,-100,0), (0,100,0), 25)
    Cylinder((0,0,-100), (0,0,100), 25)
Fillet(12, [e for e in Edges() if e.boundingBox().min.z > 0])

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occmodel as Python alternative to openscad

The object, visualized in meshlab with 'xray' shader

Recently I ran across occmodel. Its biggest prerequisite, OpenCASCADE, is packaged in Debian Stretch as "liboce"; the rest are fairly lightweight (though compiled) extensions.

So I gave it a try for a small piece, little extension legs for a dish drainer. I plan to add two leg extensions to the antisinkward legs of the dish drainer, to improve its draining behavior. So one key feature is that the captured leg will end up being at an angle to the counter.

My python program has a few "configurables" near the top, inner and outer diameters, heights, and the distance between the legs. From this, I do some basic operations (difference of two cylinders, rotation of that intermediate solid to represent the required angle, then intersection between it and a large cube that represents the half-space 'above the countertop'. Then, just to show I can do something openscad can't do trivially, I fillet every edge by 1mm.

from __future__ import division
from math import *
from geotools import *
from tostl import occ_to_stl # a little homebrew module
import os

IN=25.4

dist = 12*IN
dia = .7*IN
od = dia + .25*IN
elevation = 1*IN
engagement = 1*IN

angle = atan2(elevation, dist)

outer = Solid().createCylinder((0,0,-elevation), (0,0,elevation+engagement),od/2)
inner = Solid().createCylinder((0,0,elevation), (0,0,100),dia/2)
outer.cut(inner)
outer.rotate(angle, (1,0,0))
b = Solid().createBox((-100,-100,0), (100,100, 100))
outer.common(b)
outer.fillet(1)
with open("drainboots.stl.new", "wb") as f: occ_to_stl(outer, f)
os.rename("drainboots.stl.new", "drainboots.stl")

(I couldn't get a screenshot that did the part much justice)

What is the gap between here and a real OpenSCAD alternative?

First, I'm not confident of the robustness of liboce; I saw some segfaults and since they were dependent on specific numbers in the geometry I am tempted to assign blame there and not to the occmodel wrapper. (not that OpenSCAD is beyond reproach here)

Second, it needs an integrated environment like OpenSCAD has. Maybe one of the python "notebook" interfaces would be suitable for this.

Third, it needs less boilerplate.

Fourth, it needs broad buy-in by users of sites like thingiverse, because OpenSCAD has massive momentum behind it.

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MP Select Mini: Heated Bed Repair

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The new cable chain

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My internet usage would fit on six 33.6kb/s modems on average

Monoprice Select 3D Printer Review (part 4)

Monoprice Select 3D Printer Review (part 3)

Time Reversed Cosmology

Monoprice Select 3D Printer Review (part 2)

Three years of Prius (2013) fuel efficiency

Monoprice Select Mini 3D Printer Review

Monoprice Select Mini, sight unseen