README-NIX.md, .gitignore for corpus gen

.gitignore

@@ -90,3 +90,4 @@ Irssi-Dist
 setup.cfg
 *.egg-info
 result
+corpus/

README-NIX.md

@@ -0,0 +1,175 @@
+# Building Irssi with Nix
+
+This project includes a Nix flake for reproducible builds and development environments.
+
+## Prerequisites
+
+- [Nix](https://nixos.org/download.html) with flakes enabled
+
+To enable flakes, add to `~/.config/nix/nix.conf`:
+```
+experimental-features = nix-command flakes
+```
+
+## Quick Start
+
+```bash
+# Build irssi
+nix build
+
+# Run irssi
+./result/bin/irssi
+
+# Enter development shell
+nix develop
+```
+
+## Available Packages
+
+| Package | Description |
+|---------|-------------|
+| `irssi` (default) | Full build with Perl scripting and proxy support |
+| `irssi-minimal` | Build without Perl scripting support |
+| `fuzz` | Fuzz targets with AddressSanitizer + UndefinedBehaviorSanitizer |
+| `fuzz-asan` | Fuzz targets with AddressSanitizer only |
+| `fuzz-ubsan` | Fuzz targets with UndefinedBehaviorSanitizer only |
+
+### Building Packages
+
+```bash
+# Build default (full irssi)
+nix build
+
+# Build minimal variant
+nix build .#irssi-minimal
+
+# Build fuzzers (recommended: ASan + UBSan)
+nix build .#fuzz
+
+# Build fuzzers with only ASan (faster)
+nix build .#fuzz-asan
+```
+
+## Development Shells
+
+| Shell | Description |
+|-------|-------------|
+| `default` | Standard development with gcc, gdb, valgrind |
+| `fuzz` | Fuzzing development with clang + libFuzzer |
+
+### Using Development Shells
+
+```bash
+# Standard development
+nix develop
+meson setup Build
+ninja -C Build
+ninja -C Build test
+
+# Fuzzing development
+nix develop .#fuzz
+meson setup Build-fuzz -Dwith-perl=no -Dwithout-textui=yes -Dwith-fuzzer=yes
+ninja -C Build-fuzz
+```
+
+## Fuzzing
+
+The project includes four fuzz targets built with [libFuzzer](https://llvm.org/docs/LibFuzzer.html):
+
+| Fuzzer | Tests |
+|--------|-------|
+| `irssi-fuzz` | Text formatting (`printtext_string()`) |
+| `server-fuzz` | IRC protocol message parsing |
+| `event-get-params-fuzz` | IRC event parameter parsing |
+| `theme-load-fuzz` | Theme file loading |
+
+### Building Fuzzers
+
+```bash
+# Build with ASan + UBSan (recommended for finding bugs)
+nix build .#fuzz
+
+# Build with ASan only (faster execution)
+nix build .#fuzz-asan
+```
+
+### Running Fuzzers
+
+```bash
+# Basic fuzzing (creates corpus automatically)
+./result/bin/irssi-fuzz corpus/irssi-fuzz/
+
+# With dictionary (recommended for server-fuzz)
+./result/bin/server-fuzz -dict=src/fe-fuzz/tokens.txt corpus/server-fuzz/
+
+# Limit number of runs
+./result/bin/irssi-fuzz -runs=10000 corpus/irssi-fuzz/
+
+# Parallel fuzzing (use multiple cores)
+./result/bin/server-fuzz -fork=4 -dict=src/fe-fuzz/tokens.txt corpus/server-fuzz/
+
+# Ignore memory leaks to focus on crashes
+./result/bin/server-fuzz -detect_leaks=0 corpus/server-fuzz/
+```
+
+### Seed Corpus
+
+Initial seed inputs are provided in `fuzz-corpora/`:
+
+```bash
+# Copy seeds to corpus directories
+mkdir -p corpus/irssi-fuzz corpus/server-fuzz corpus/event-get-params-fuzz corpus/theme-load-fuzz
+cp fuzz-corpora/irssi-fuzz/* corpus/irssi-fuzz/
+cp fuzz-corpora/server-fuzz/* corpus/server-fuzz/
+cp fuzz-corpora/event-get-params-fuzz/* corpus/event-get-params-fuzz/
+cp fuzz-corpora/theme-load-fuzz/* corpus/theme-load-fuzz/
+```
+
+### Reproducing Crashes
+
+When a fuzzer finds a crash, it saves the input to a file:
+
+```bash
+# Reproduce a crash
+./result/bin/server-fuzz crash-<hash>
+
+# Get more details with symbolized stack trace
+ASAN_OPTIONS=symbolize=1 ./result/bin/server-fuzz crash-<hash>
+```
+
+### Fuzzer Input Formats
+
+- **irssi-fuzz**: Arbitrary text, may contain irssi format codes (`%B`, `%U`, etc.)
+- **server-fuzz**: Byte 0 selects prefix mode, remaining bytes are `\r\n`-separated IRC messages
+- **event-get-params-fuzz**: Byte 0 selects parsing mode (0-7), remaining bytes are parameters
+- **theme-load-fuzz**: irssi theme file format
+
+## Continuous Integration
+
+To check that everything builds:
+
+```bash
+nix flake check
+```
+
+## Troubleshooting
+
+### UBSan warnings about function pointer types
+
+The warning about `signals.c` function pointer types is expected:
+```
+runtime error: call to function through pointer to incorrect function type
+```
+
+This is due to irssi's dynamic signal dispatch system and is a known pattern in the codebase.
+
+### Fuzzer stops immediately
+
+Ensure the corpus directory exists:
+```bash
+mkdir -p corpus/irssi-fuzz
+```
+
+### Build fails with "perl not found"
+
+Perl is required even for minimal/fuzzer builds (for generating help files). The Nix flake handles this automatically.