"This is the first book that promises to tell the deep, dark secrets of computer arithmetic, and it delivers in spades. It contains every trick I knew plus many, many more. A godsend for library developers, compiler writers, and lovers of elegant hacks, it deserves a spot on your shelf right next to Knuth."
--Josh Bloch"When I first saw the title, I figured that the book must be either a cookbook for breaking into computers (unlikely) or some sort of compendium of little programming tricks. It's the latter, but it's thorough, almost encyclopedic, in its coverage."
--Guy SteeleThese are the timesaving techniques relished by computer hackers--those devoted and persistent code developers who seek elegant and efficient ways to build better software. The truth is that much of the computer programmer's job involves a healthy mix of arithmetic and logic. In Hacker's Delight, veteran programmer Hank Warren shares the tricks he has collected from his considerable experience in the worlds of application and system programming. Most of these techniques are eminently practical, but a few are included just because they are interesting and unexpected. The resulting work is an irresistible collection that will help even the most seasoned programmers better their craft.
Topics covered include:
- A broad collection of useful programming tricks
- Small algorithms for common tasks
- Power-of-2 boundaries and bounds checking
- Rearranging bits and bytes
- Integer division and division by constants
- Some elementary functions on integers
- Gray code
- Hilbert's space-filling curve
- And even formulas for prime numbers!
This book is for anyone who wants to create efficient code. Hacker's Delight will help you learn to program at a higher level--well beyond what is generally taught in schools and training courses--and will advance you substantially further than is possible through ordinary self-study alone.
0201914654B06272002 Excerpt. © Reprinted by permission. All rights reserved.
Caveat Emptor: The cost of software maintenance
increases with the square of the programmer's creativity.First Law of Programmer Creativity, Robert D. Bliss, 1992
This is a collection of small programming tricks that I have come across over many years. Most of them will work only on computers that represent integers in two's-complement form. Although a 32-bit machine is assumed when the register length is relevant, most of the tricks are easily adapted to machines with other register sizes.
This book does not deal with large tricks such as sophisticated sorting and compiler optimization techniques. Rather, it deals with small tricks that usually involve individual computer words or instructions, such as counting the number of 1-bits in a word. Such tricks often use a mixture of arithmetic and logical instructions.
It is assumed throughout that integer overflow interrupts have been masked off, so they cannot occur. C, Fortran, and even Java programs run in this environment, but Pascal and ADA users beware!
The presentation is informal. Proofs are given only when the algorithm is not obvious, and sometimes not even then. The methods use computer arithmetic, "floor" functions, mixtures of arithmetic and logical operations, and so on. Proofs in this domain are often difficult and awkward to express.
To reduce typographical errors and oversights, many of the algorithms have been executed. This is why they are given in a real programming language, even though, like every computer language, it has some ugly features. C is used for the high-level language because it is widely known, it allows the straightforward mixture of integer and bit-string operations, and C compilers that produce high-quality object code are available.
Occasionally, machine language is used. It employs a three-address format, mainly for ease of readability. The assembly language used is that of a fictitious machine that is representative of today's RISC computers.
Branch-free code is favored. This is because on many computers, branches slow down instruction fetching and inhibit executing instructions in parallel. Another problem with branches is that they may inhibit compiler optimizations such as instruction scheduling, commoning, and register allocation. That is, the compiler may be more effective at these optimizations with a program that consists of a few large basic blocks rather than many small ones.
The code sequences also tend to favor small immediate values, comparisons to zero (rather than to some other number), and instruction-level parallelism. Although much of the code would become more concise by using table lookups (from memory), this is not often mentioned. This is because loads are becoming more expensive relative to arithmetic instructions, and the table lookup methods are often not very interesting (although they are often practical). But there are exceptional cases.
Finally, I should mention that the term "hacker" in the title is meant in the original sense of an aficionado of computers--someone who enjoys making computers do new things, or do old things in a new and clever way. The hacker is usually quite good at his craft, but may very well not be a professional computer programmer or designer. The hacker's work may be useful or may be just a game. As an example of the latter, more than one determined hacker has written a program which, when executed, writes out an exact copy of itself1. This is the sense in which we use the term "hacker." If you're looking for tips on how to break into someone else's computer, you won't find them here.H. S. Warren, Jr.
Yorktown, New York
February 2002
1. The shortest such program written in C, known to the present author, is by Vlad Taeerov and Rashit Fakhreyev and is 64 characters in length: main(a){printf(a,34,a="main(a){printf(a,34,a=%c%s%c,34);}",34);}
0201914654P08282002
Excerpt. © Reprinted by permission. All rights reserved.
Caveat Emptor: The cost of software maintenance
increases with the square of the programmer's creativity.
First Law of Programmer Creativity, Robert D. Bliss, 1992 This is a collection of small programming tricks that I have come across over many years. Most of them will work only on computers that represent integers in two's-complement form. Although a 32-bit machine is assumed when the register length is relevant, most of the tricks are easily adapted to machines with other register sizes.
This book does not deal with large tricks such as sophisticated sorting and compiler optimization techniques. Rather, it deals with small tricks that usually involve individual computer words or instructions, such as counting the number of 1-bits in a word. Such tricks often use a mixture of arithmetic and logical instructions.
It is assumed throughout that integer overflow interrupts have been masked off, so they cannot occur. C, Fortran, and even Java programs run in this environment, but Pascal and ADA users beware!
The presentation is informal. Proofs are given only when the algorithm is not obvious, and sometimes not even then. The methods use computer arithmetic, "floor" functions, mixtures of arithmetic and logical operations, and so on. Proofs in this domain are often difficult and awkward to express.
To reduce typographical errors and oversights, many of the algorithms have been executed. This is why they are given in a real programming language, even though, like every computer language, it has some ugly features. C is used for the high-level language because it is widely known, it allows the straightforward mixture of integer and bit-string operations, and C compilers that produce high-quality object code are available.
Occasionally, machine language is used. It employs a three-address format, mainly for ease of readability. The assembly language used is that of a fictitious machine that is representative of today's RISC computers.
Branch-free code is favored. This is because on many computers, branches slow down instruction fetching and inhibit executing instructions in parallel. Another problem with branches is that they may inhibit compiler optimizations such as instruction scheduling, commoning, and register allocation. That is, the compiler may be more effective at these optimizations with a program that consists of a few large basic blocks rather than many small ones.
The code sequences also tend to favor small immediate values, comparisons to zero (rather than to some other number), and instruction-level parallelism. Although much of the code would become more concise by using table lookups (from memory), this is not often mentioned. This is because loads are becoming more expensive relative to arithmetic instructions, and the table lookup methods are often not very interesting (although they are often practical). But there are exceptional cases.
Finally, I should mention that the term "hacker" in the title is meant in the original sense of an aficionado of computers--someone who enjoys making computers do new things, or do old things in a new and clever way. The hacker is usually quite good at his craft, but may very well not be a professional computer programmer or designer. The hacker's work may be useful or may be just a game. As an example of the latter, more than one determined hacker has written a program which, when executed, writes out an exact copy of itself1. This is the sense in which we use the term "hacker." If you're looking for tips on how to break into someone else's computer, you won't find them here.
H. S. Warren, Jr.
Yorktown, New York
February 2002
1. The shortest such program written in C, known to the present author, is by Vlad Taeerov and Rashit Fakhreyev and is 64 characters in length: main(a){printf(a,34,a="main(a){printf(a,34,a=%c%s%c,34);}",34);}
0201914654P08282002 
