CS Foundations

A blog series

Computer Science Foundations Interviewers Assume

The computer science underneath your code: how programs run, memory and CPU caches, floating point, Unicode, binary and bit tricks, probability, hard problems and string searching, explained in plain English with runnable examples.

Started 
2026
Lessons 
8
Reading 
~2 h

Contents

Every lesson has the same steps: what it is, how it works, a real example, common mistakes, then interview questions with answers you can say out loud.

  1. Lesson 00How a Computer Runs Your ProgramFrom source code to machine code: compiling, interpreting and JIT, the stack and the heap, function calls, stack overflow and garbage collection.20 min
  2. Lesson 01Memory, CPU Caches and Why Array Order MattersThe memory hierarchy, cache lines, locality, and a timing experiment that walks a matrix row by row and column by column.15 min
  3. Lesson 02Floating Point: Why 0.1 + 0.2 Is Not 0.3IEEE 754 doubles with bit-level demos, rounding, safe integers, comparing floats and handling money.16 min
  4. Lesson 03Unicode, UTF-8 and Strings That LieCode points, UTF-16 code units, bytes and grapheme clusters, why emoji lengths surprise you, normalisation and safe truncation.20 min
  5. Lesson 04Binary, Endianness and Bit TricksReading binary and hex, two's complement, JavaScript's 32-bit bitwise operators, masks, endianness and DataView.17 min
  6. Lesson 05Combinatorics and Probability for InterviewsCounting rules, permutations and combinations, the birthday paradox, expected value, Bayes in plain words, Fisher-Yates and reservoir sampling.17 min
  7. Lesson 06P, NP and Hard Problems: What to Say in an InterviewComplexity classes in plain words, NP-complete examples, and what to do when a problem is hard: exact for small inputs, approximation and heuristics.15 min
  8. Lesson 07String Searching: Naive, KMP, Rabin-Karp and ZThe naive worst case, KMP with a traced failure function, Rabin-Karp collisions, the Z-array and a tiny Aho-Corasick, with local timings.14 min

Glossary

17 terms used in this series. Inside any lesson, hover or tap a dotted term to see its meaning without leaving the page. Browse all terms →