From 032ca56bbf5ead678d21ca3fa83577662f5f0789 Mon Sep 17 00:00:00 2001
From: Humza Shahid <humzasaur@gmail.com>
Date: Sun, 28 Sep 2025 22:01:44 +0100
Subject: [PATCH] add initial implementation of compiling a regex string to an
 NFA

---
 fcore/search-list/nfa.sml | 112 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 fcore/search-list/nfa.sml

diff --git a/fcore/search-list/nfa.sml b/fcore/search-list/nfa.sml
new file mode 100644
index 0000000..282d0f8
--- /dev/null
+++ b/fcore/search-list/nfa.sml
@@ -0,0 +1,112 @@
+structure Nfa =
+struct
+  datatype regex =
+    CONCAT of regex * regex
+  | CHAR_LITERAL of char
+  | ALTERNATION of regex * regex
+  | ZERO_OR_ONE of regex
+  | ZERO_OR_MORE of regex
+  | ONE_OR_MORE of regex
+  | GROUP of regex
+
+  val groupLevel = 1
+  val postfixLevel = 2
+  val concatLevel = 3
+  val altLevel = 4
+
+  local
+    fun loop (pos, str, openParens, closeParens) =
+      if pos = String.size str then
+        pos
+      else
+        case String.sub (str, pos) of
+          #"(" => loop (pos + 1, str, openParens + 1, closeParens)
+        | #")" =>
+            if closeParens + 1 = openParens then pos
+            else loop (pos + 1, str, openParens, closeParens + 1)
+        | _ => loop (pos + 1, str, openParens, closeParens)
+  in
+    fun getRightParenIdx (pos, str) = loop (pos, str, 1, 0)
+  end
+
+  fun helpClimb (pos, str, lhs, level) =
+    if pos = String.size str then
+      (pos, lhs)
+    else
+      case String.sub (str, pos) of
+        #")" => (pos + 1, lhs)
+      | #"(" =>
+          if level < groupLevel then
+            (pos, lhs)
+          else
+            let
+              val groupEndIdx = getRightParenIdx (pos + 1, str)
+              val substr = String.substring
+                (str, pos + 1, groupEndIdx - pos - 1)
+              val rhs = climb substr
+              val rhs = GROUP rhs
+              val result = CONCAT (lhs, rhs)
+            in
+              helpClimb (groupEndIdx + 1, str, result, groupLevel)
+            end
+      | #"|" =>
+          if level < altLevel then
+            (pos, lhs)
+          else
+            let
+              val chr = String.sub (str, pos + 1)
+              val chr = CHAR_LITERAL chr
+              val (pos, rhs) = helpClimb (pos + 2, str, chr, altLevel)
+              val result = ALTERNATION (lhs, rhs)
+            in
+              (pos, result)
+            end
+      | #"?" =>
+          if level < postfixLevel then
+            (pos, lhs)
+          else
+            let val lhs = ZERO_OR_ONE lhs
+            in helpClimb (pos + 1, str, lhs, postfixLevel)
+            end
+      | #"*" =>
+          if level < postfixLevel then
+            (pos, lhs)
+          else
+            let val lhs = ZERO_OR_MORE lhs
+            in helpClimb (pos + 1, str, lhs, postfixLevel)
+            end
+      | #"+" =>
+          if level < postfixLevel then
+            (pos, lhs)
+          else
+            let val lhs = ONE_OR_MORE lhs
+            in helpClimb (pos + 1, str, lhs, postfixLevel)
+            end
+      | chr =>
+          if level < concatLevel then
+            (pos, lhs)
+          else
+            let
+              val chr = CHAR_LITERAL chr
+              val (pos, rhs) = helpClimb (pos + 1, str, chr, concatLevel)
+              val result = CONCAT (lhs, rhs)
+            in
+              (pos, result)
+            end
+
+  and loop (pos, str, ast) =
+    if pos = String.size str then
+      ast
+    else
+      let val (pos, ast) = helpClimb (pos, str, ast, altLevel)
+      in loop (pos, str, ast)
+      end
+
+  and climb str =
+    let
+      val chr = String.sub (str, 0)
+      val chr = CHAR_LITERAL chr
+    in
+      loop (1, str, chr)
+    end
+end