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Prechelt Phone Number Encoding

This project implements the phone number encoding described by Lutz Prechelt in his article for the COMMUNICATIONS OF THE ACM (October 1999/Vol. 42, No. 10), Comparing Java vs. C/C++ Efficiency Differences to Interpersonal Differences, later re-used for a more extended paper, An empirical comparisonofC, C++, Java,Perl, Python, Rexx, and Tclfor asearch/string-processing program from March, 2000.

The same problem was later used by Ron Garret (aka Erann Gat) on Lisp as an alternative to Java, also from the year 2000.

The instructions given to participants of the latter study (which was slightly adapted from the original paper, and used to implement my own solution in this repository) can be found at

Peter Norvig, after finding out about the Lisp paper, wrote his own Lisp program to solve the problem.

His solution is included in this repository at src/lisp/main.lisp as a baseline for performance measurements (see

Structure of this repository

The solution I first came up with is in src/java/

Later, I ported Norvig's Lisp solution to Java (src/java/ and Rust (src/rust/phone_encoder/src/

In order to be able to benchmark the programs, I created a few utilities in Java and Rust:

To run the benchmarks, a shell script,, is used.

Compiling and running


The benchmarks currently require the following tools:

  • Java 16+.
  • Rust and Cargo (tested with rustc 1.53.0 (53cb7b09b 2021-06-17)).
  • SBCL, Steel Bank Common Lisp compiler.


Run this command to compile the Java solutions:

$ javac src/java/*.java -d build

To compile the utilities, run:

$ javac src/java/util/*.java -d build


Compile the Rust solution:

$ cd src/rust/phone_encoder
$ cargo build --release

Compile the Benchmark runner:

$ cd src/rust/benchmark_runner
$ cargo build --release


No compilation is required.



$ java -cp build Main dictionary.txt input.txt

For all programs, you can use the smaller dictionary at tests/words.txt and input file tests/numbers.txt to print just a few results.


$ cp src/rust/phone_encoder/target/release/phone_encoder .
$ ./phone_encoder dictionary.txt input.txt


$ sbcl --script src/lisp/main.lisp dictionary.txt input.txt


To verify that your program works correctly execute it with arguments dictionary.txt input.txt, piping its output to a file, say prog_out.txt, then run:

$ diff -q <(sort prog_out.txt) <(sort output.txt)

If diff exits with 0 and does not print anything, your program is good.

The benchmarks generate input files of different sizes by running my Java utility. If you want to try your solution with larger inputs, try this, for example, to generate 50,000 random phone numbers:

$ java -cp build/util util.GeneratePhoneNumbers 50000 > phones_50_000.txt

To partially check your solution works with the larger inputs (the checker cannot guarantee that the digit-substitution algorithm is perfectly correct), run my Java utility as follows:

$ java -cp build/util util.OutputChecker dictionary.txt program_output.txt


I've published the benchmarks results on a Google Docs Spreadsheet.

To run it yourself, checkout this repository and run:

See the requirements above first.


Adding a new program to the benchmarks

I don't currently intend to accept pull requests to this repository because I used the programs in this repository on an article I wrote on my website and I don't want it to become stale.

However, feel free to fork it and modify it as you see fit!

  1. Add your program to the appropriate folder under src/lang/ (e.g. src/lisp/main.lisp).
  2. If it requires a new compiler, update's Requirements section.
  3. Edit as follows:
    • if it requires compilation, add the compilation command after the echo "Compiling Rust sources" block.
    • add the command to run your program to the COMMANDS array.

Solutions in other languages

I've found the following other solutions to Prechelt's phone number encoding so far:

This repository includes also Dart and Julia solutions in the dart and julia branches, respectively.


Comparison between Java and Common Lisp solutions to a phone-encoding problem described by Prechelt






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