frontend: Add lexer+parser groundwork

Signed-off-by: Ian M. <ian@aptel.org>

6 files changed, 223 insertions, 1 deletions

diff --git a/core/atas.c b/core/atas.c
index 9585a26..623468f 100644
--- a/core/atas.c
+++ b/core/atas.c
@@ -5,7 +5,8 @@
 
 #include <stdio.h>
 #include <unistd.h>
-#include "atas/state.h"
+#include <atas/state.h>
+#include <atas/parser.h>
 
 /*
  * Display help options
@@ -39,6 +40,13 @@ atas_assemble(const char *in_file)
         return;
     }
 
+    error = atas_parse(&state);
+    if (error < 0) {
+        printf("failured occured in %s\n", in_file);
+        atas_state_destroy(&state);
+        return;
+    }
+
     atas_state_destroy(&state);
 }
 
diff --git a/core/lexer.c b/core/lexer.c
new file mode 100644
index 0000000..019724c
--- /dev/null
+++ b/core/lexer.c
@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+#include <atas/lexer.h>
+#include <stdio.h>
+#include <stdbool.h>
+
+/*
+ * Returns true if the given char is a whitespace
+ *
+ * @c:  Character to compare against a set of whitespace
+ */
+static inline bool
+lexer_is_ws(char c)
+{
+    switch (c) {
+    case '\n':
+    case '\f':
+    case '\t':
+    case '\r':
+    case ' ':
+        return true;
+    }
+
+    return false;
+}
+
+/*
+ * Scan a single byte from the input source file
+ *
+ * @state:   Assembler state
+ * @skip_ws: If true, skip whitespaces
+ *
+ * Returns the scanned byte on success, returns '\0'
+ * on failure.
+ */
+static char
+lexer_scan_byte(struct atas_state *state, bool skip_ws)
+{
+    char c;
+
+    if (state == NULL) {
+        return '\0';
+    }
+
+    c = fgetc(state->in_file);
+    while (lexer_is_ws(c) && skip_ws) {
+        if ((c = fgetc(state->in_file)) == EOF) {
+            return '\0';
+        }
+    }
+
+    return c;
+}
+
+int
+atas_lexer_scan(struct atas_state *state, struct atas_token *token)
+{
+    char c;
+
+    if (state == NULL || token == NULL) {
+        return -1;
+    }
+
+    c = lexer_scan_byte(state, true);
+    if (c == '\0') {
+        return -1;
+    }
+
+    switch (c) {
+    case ',':
+        token->type = TT_COMMA;
+        token->c = c;
+        return 0;
+    }
+
+    return -1;
+}
diff --git a/core/parser.c b/core/parser.c
new file mode 100644
index 0000000..6cf0a4e
--- /dev/null
+++ b/core/parser.c
@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+#include <stdint.h>
+#include <stddef.h>
+#include <atas/lexer.h>
+
+/* Symbolic token */
+#define SYMTOK(TOK) \
+    "[" TOK "]"
+
+/* Literal token */
+#define LITTOK(TOK) \
+    "'" TOK "'"
+
+/*
+ * Safe macro to index toktab
+ */
+#define TOK_STR(TT)     \
+    ((TT) >= TT_MAX)    \
+        ? "bad"         \
+        : toktab[(TT)]
+
+/*
+ * Lookup table to convert token constants into
+ * strings.
+ */
+static const char *toktab[] = {
+    [TT_NONE]       = SYMTOK("NONE"),
+    [TT_COMMA]      = LITTOK(",")
+};
+
+int
+atas_parse(struct atas_state *state)
+{
+    int error;
+    struct atas_token tok;
+
+    if (state == NULL) {
+        return -1;
+    }
+
+    for (;;) {
+        error = atas_lexer_scan(state, &tok);
+        if (error < 0) {
+            break;
+        }
+
+        printf("got token %s\n", TOK_STR(tok.type));
+    }
+
+    return 0;
+}
diff --git a/head/atas/lexer.h b/head/atas/lexer.h
new file mode 100644
index 0000000..5944825
--- /dev/null
+++ b/head/atas/lexer.h
@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+#ifndef ATAS_LEXER_H
+#define ATAS_LEXER_H 1
+
+#include <atas/token.h>
+#include <atas/state.h>
+
+/*
+ * Scan for a single lexical element within a program
+ *
+ * @state:  ATAS state
+ * @token:  Token result is written here
+ *
+ * Returns zero on success
+ */
+int atas_lexer_scan(struct atas_state *state, struct atas_token *token);
+
+#endif  /* !ATAS_LEXER_H */
diff --git a/head/atas/parser.h b/head/atas/parser.h
new file mode 100644
index 0000000..7e410c8
--- /dev/null
+++ b/head/atas/parser.h
@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+#ifndef ATAS_PARSER_H
+#define ATAS_PARSER_H 1
+
+#include <atas/state.h>
+
+/*
+ * Begin parsing the input source file
+ *
+ * @state:  ATAS state machine
+ *
+ * Returns zero on success
+ */
+int atas_parse(struct atas_state *state);
+
+#endif  /* !ATAS_PARSER_H */
diff --git a/head/atas/token.h b/head/atas/token.h
new file mode 100644
index 0000000..1346eba
--- /dev/null
+++ b/head/atas/token.h
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+#ifndef ATAS_TOKEN_H
+#define ATAS_TOKEN_H 1
+
+#include <sys/types.h>
+#include <stdint.h>
+#include <stddef.h>
+
+/*
+ * Token type
+ */
+typedef enum {
+    TT_NONE,        /* [NONE] */
+    TT_COMMA,       /* ',' */
+    TT_MAX
+} atas_tt_t;
+
+/*
+ * Represents a single lexical element within a program
+ *
+ * @type:  Token type
+ * @c:     Character associated with token
+ * @v:     Signed value associated with token
+ */
+struct atas_token {
+    atas_tt_t type;
+    union {
+        char c;
+        ssize_t v;
+    };
+};
+
+#endif  /* !ATAS_TOKEN_H */