From f796560cb4de0b6de118c00be54c58206bad5b60 Mon Sep 17 00:00:00 2001
From: Jonas Knobloch <jonas.knobloch@t-online.de>
Date: Tue, 24 Mar 2026 15:37:50 +0100
Subject: Expose finite state automaton

---
 tokenizer/bpe/split/fsa.go | 230 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 230 insertions(+)
 create mode 100644 tokenizer/bpe/split/fsa.go

(limited to 'tokenizer/bpe/split')

diff --git a/tokenizer/bpe/split/fsa.go b/tokenizer/bpe/split/fsa.go
new file mode 100644
index 0000000..8c95c89
--- /dev/null
+++ b/tokenizer/bpe/split/fsa.go
@@ -0,0 +1,230 @@
+package split
+
+import (
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+const (
+	RuneUnicode32 = iota
+	RuneWhitespaceNotUnicode32
+	RuneLetter
+	RuneNumber
+	RuneOther
+	StateInitial
+	StateU32
+	StateWhitespaceNotUnicode32
+	StateLetter
+	StateNumber
+	StateOther
+	StateWhitespaceLookAhead
+)
+
+type FSA struct {
+	state  int
+	input  []rune
+	static []string
+}
+
+func NewFSA() *FSA {
+	return &FSA{
+		state:  StateInitial,
+		input:  make([]rune, 0),
+		static: []string{"'s", "'t", "'re", "'m", "'ll", "'d"},
+	}
+}
+
+func (f *FSA) Reset() {
+	f.state = StateInitial
+	f.input = make([]rune, 0)
+}
+
+func (f *FSA) Read(next rune) bool {
+	var r int
+
+	if next == 32 {
+		r = RuneUnicode32
+	} else if unicode.IsSpace(next) {
+		r = RuneWhitespaceNotUnicode32
+	} else if unicode.IsLetter(next) {
+		r = RuneLetter
+	} else if unicode.IsNumber(next) {
+		r = RuneNumber
+	} else {
+		r = RuneOther
+	}
+
+	if f.state == StateInitial {
+		switch r {
+		case RuneUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateU32
+
+			break
+		case RuneWhitespaceNotUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceNotUnicode32
+
+			break
+		case RuneLetter:
+			f.input = append(f.input, next)
+			f.state = StateLetter
+
+			break
+		case RuneNumber:
+			f.input = append(f.input, next)
+			f.state = StateNumber
+
+			break
+		default:
+			f.input = append(f.input, next)
+			f.state = StateOther
+		}
+	} else if f.state == StateU32 {
+		switch r {
+		case RuneUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceLookAhead
+
+			break
+		case RuneWhitespaceNotUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceLookAhead
+
+			break
+		case RuneLetter:
+			f.input = append(f.input, next)
+			f.state = StateLetter
+
+			break
+		case RuneNumber:
+			f.input = append(f.input, next)
+			f.state = StateNumber
+
+			break
+		default:
+			f.input = append(f.input, next)
+			f.state = StateOther
+		}
+	} else if f.state == StateWhitespaceNotUnicode32 {
+		switch r {
+		case RuneUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceLookAhead
+
+			break
+		case RuneWhitespaceNotUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceLookAhead
+
+			break
+		default:
+			return false
+		}
+	} else if f.state == StateNumber {
+		switch r {
+		case RuneNumber:
+			f.input = append(f.input, next)
+			f.state = StateNumber
+
+			break
+		default:
+			return false
+		}
+	} else if f.state == StateLetter {
+		switch r {
+		case RuneLetter:
+			f.input = append(f.input, next)
+			f.state = StateLetter
+
+			break
+		default:
+			return false
+		}
+	} else if f.state == StateOther {
+		switch r {
+		case RuneOther:
+			f.input = append(f.input, next)
+			f.state = StateOther
+
+			break
+		default:
+			return false
+		}
+	} else if f.state == StateWhitespaceLookAhead {
+		switch r {
+		case RuneUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceLookAhead
+
+			break
+		case RuneWhitespaceNotUnicode32:
+			f.input = append(f.input, next)
+			f.state = StateWhitespaceLookAhead
+
+			break
+		default:
+			return false
+		}
+	} else {
+		panic("invalid state")
+	}
+
+	return true
+}
+
+func (f *FSA) FindAll(s string) []string {
+	var findAll func(runes []rune, matches []string) []string
+
+	findAll = func(runes []rune, matches []string) []string {
+		s = string(runes)
+
+		for _, v := range f.static {
+			if strings.HasPrefix(s, v) {
+				matches = append(matches, v)
+				runes = runes[utf8.RuneCountInString(v):]
+
+				if len(runes) == 0 {
+					return matches
+				}
+
+				return findAll(runes, matches)
+			}
+		}
+
+		for i, r := range runes {
+			ok := f.Read(r)
+
+			if !ok {
+				if f.state == StateInitial {
+					return matches
+				}
+
+				if f.state == StateWhitespaceLookAhead {
+					matches = append(matches, string(f.input[:len(f.input)-1]))
+
+					f.Reset()
+
+					return findAll(runes[i-1:], matches)
+				}
+
+				matches = append(matches, string(f.input))
+
+				f.Reset()
+
+				return findAll(runes[i:], matches)
+			}
+		}
+
+		if len(f.input) > 0 {
+			matches = append(matches, string(f.input))
+		}
+
+		return matches
+	}
+
+	defer f.Reset()
+
+	return findAll([]rune(s), make([]string, 0))
+}
-- 
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