Most programs are too complicated — that is, more complex than they need to be to solve their problems efficiently. Why? Mostly it’s because of bad design, but I will skip that issue here because it’s a big one. But programs are often complicated at the microscopic level, and that is something I can address here.

• Rule 1. You can’t tell where a program is going to spend its time. Bottlenecks occur in surprising places, so don’t try to second guess and put in a speed hack until you’ve proven that’s where the bottleneck is.
• Rule 2. Measure. Don’t tune for speed until you’ve measured, and even then don’t unless one part of the code overwhelms the rest.
• Rule 3. Fancy algorithms are slow when n is small, and n is usually small. Fancy algorithms have big constants. Until you know that n is frequently going to be big, don’t get fancy. (Even if_n_ does get big, use Rule 2 first.) For example, binary trees are always faster than splay trees for workaday problems.
• Rule 4. Fancy algorithms are buggier than simple ones, and they’re much harder to implement. Use simple algorithms as well as simple data structures.
  The following data structures are a complete list for almost all practical programs: array, linked list, hash table, binary tree.
  Of course, you must also be prepared to collect these into compound data structures. For instance, a symbol table might be implemented as a hash table containing linked lists of arrays of characters.
• Rule 5. Data dominates. If you’ve chosen the right data structures and organized things well, the algorithms will almost always be self-evident. Data structures, not algorithms, are central to programming. (See Brooks p. 102.)
• Rule 6. There is no Rule 6.

-- Rob Pike, Programming in C: Complexity