#ifndef HA_SEXP_H
#define HA_SEXP_H

/**
 * Prints an s-expression to stdout. The spec string is a list of format
 * specifiers describing the structure of the s-expression. Unlike the stdio
 * family of printing functions, % is not used - each character is interpreted
 * directly and there is no means provided for emitting additional unstructured
 * text. The value of each specifier is provided in the remaining arguments.
 * The following format specifiers are used:
 *
 * a: atom (string: alphanumeric, plus _ and -, no spaces)
 * s: string
 * d: integer
 * f: floating point number
 * (, ): list
 * ' ', '\n', '\t': copied to output
 *
 * The return value is the number of bytes written.
 *
 * To write a variable-length list, omit the closing ) from the format string,
 * then call printf several times to write each member. Finish with a final ).
 */
int ha_sexp_printf(const char *spec, ...);

/**
 * Scans an s-expression from stdin which matches the given format specifier,
 * and populates the remaining arguments with the values found in the
 * s-expression. The same format specifiers as ha_sexp_printf are supported, but
 * with the following constraints:
 *
 * a, s: pass one char * argument and one size_t argument with the buffer
 * length. If the buffer length is exceeded, errno is set to ENOBUFS.
 *
 * A, S: pass one char ** argument, which must be initialized to NULL. The
 * buffer will be allocated. ENOMEM may be returned if the buffer cannot be
 * allocated. In the case of an error, your pointer will remain NULL and does
 * not need to be freed.
 *
 * In the case of invalid strings or atoms, errno is set to EINVAL.
 *
 * The return value is the number of values successfully scanned in. This
 * number includes ( and ), but does not include whitespace characters. The
 * caller SHOULD check that this number meets their expectations and should
 * treat this as an error if not.
 *
 * To scan a variable-length list, include the closing ) in your format string.
 * If the return value is equal to the number of values -1, there are additional
 * members to scan. Scan each of these additional members without the opening (,
 * but include the closing ). Continue scanning until the ) is included in the
 * number of successfully scanned values.
 */
int ha_sexp_scanf(const char *spec, ...);

#endif

#include <assert.h>
#include <ctype.h>
#include <errno.h>
#include <stdarg.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include "sexp.h"

static int
put_atom(const char *str)
{
	int i;
	for (i = 0; str[i]; ++i) {
		assert(isalnum(str[i]) || str[i] == '-' || str[i] == '_');
		putchar(str[i]);
	}
	return i;
}

static int
put_string(const char *str)
{
	int i;
	putchar('"');
	for (i = 0; str[i]; ++i) {
		switch (str[i]) {
		case '"':
			putchar('\\');
			putchar('"');
			break;
		case '\\':
			putchar('\\');
			putchar('\\');
			break;
		default:
			if (isprint(str[i])) {
				putchar(str[i]);
			} else {
				printf("\\u%4x", str[i]);
			}
			break;
		}
	}
	putchar('"');
	return i + 2;
}

int
ha_sexp_printf(const char *s, ...)
{
	va_list ap;
	va_start(ap, s);
	int l = 0;
	char c;
	while ((c = *s)) {
		++s;
		switch (c) {
		case 'a':
			l += put_atom(va_arg(ap, const char *));
			break;
		case 'd':
			l += printf("%d", va_arg(ap, int));
			break;
		case 'f':
			l += printf("%f", va_arg(ap, double));
			break;
		case 's':
			l += put_string(va_arg(ap, const char *));
			break;
		case '(':
			putchar('(');
			++l;
			continue;
		case ')':
			putchar(')');
			++l;
			break;
		case ' ':
		case '\n':
		case '\t':
			putchar(c);
			++l;
			continue;
		default:
			assert(0);
		}
		if (*s && *s != ')') {
			putchar(' ');
			++l;
		}
	}
	va_end(ap);
	return l;
}

static char *
scan_atom(char *buf, size_t sz)
{
	size_t bufsz = 1024;
	if (sz == 0) {
		assert(buf == NULL);
		buf = malloc(bufsz);
		if (!buf) {
			errno = ENOMEM;
			goto error;
		}
	}
	char c;
	while (isspace(c = getchar()) && c != -1);
	buf[0] = c;
	for (size_t i = 1; sz == 0 || i < sz; ++i) {
		if (sz == 0 && i >= bufsz) {
			char *newbuf = realloc(buf, bufsz * 2);
			if (!newbuf) {
				errno = ENOMEM;
				goto error;
			}
			bufsz *= 2;
			buf = newbuf;
		}
		c = getchar();
		if (c == -1) {
			buf[i] = '\0';
			return buf;
		}
		if (isspace(c) || c == ')') {
			ungetc(c, stdin);
			buf[i] = '\0';
			return buf;
		}
		if (!isalnum(c) && c != '-' && c != '_') {
			errno = EINVAL;
			goto error;
		}
		buf[i] = c;
	}
	errno = ENOBUFS;
error:
	if (sz == 0) {
		free(buf);
	}
	return NULL;
}

static char *
scan_str(char *buf, size_t sz)
{
	char c;
	while (isspace(c = getchar()) && c != -1);
	if (c != '"') {
		return NULL;
	}
	size_t bufsz = 1024;
	if (sz == 0) {
		assert(buf == NULL);
		buf = malloc(bufsz);
		if (!buf) {
			errno = ENOMEM;
			goto error;
		}
	}
	for (size_t i = 0; sz == 0 || i < sz; ++i) {
		if ((c = getchar()) == -1) {
			goto inval;
		}
		if (sz == 0 && i >= bufsz) {
			char *newbuf = realloc(buf, bufsz * 2);
			if (!newbuf) {
				errno = ENOMEM;
				goto error;
			}
			bufsz *= 2;
			buf = newbuf;
		}
		if (c == '"') {
			/* Success */
			buf[i] = '\0';
			return buf;
		}
		unsigned int n;
		switch (c) {
		case '\\':
			c = getchar();
			if (c == -1) {
				goto inval;
			}
			switch (c) {
			case '"':
			case '\\':
				buf[i] = c;
				break;
			case 'u':
				if (scanf("%4x", &n) != 1) {
					goto inval;
				}
				buf[i] = (char)n;
				break;
			default:
				goto inval;
			}
			break;
		default:
			buf[i] = c;
			break;
		}
	}
	errno = ENOBUFS;
	goto error;
inval:
	errno = EINVAL;
error:
	if (sz == 0) {
		free(buf);
	}
	return NULL;
}

int
ha_sexp_scanf(const char *s, ...)
{
	va_list ap;
	va_start(ap, s);
	int n = 0, r;
	char c;
	char *str, **strptr;
	size_t sz;
	while ((c = *s)) {
		++s;
		r = 0;
		switch (c) {
		case 'a':
			str = va_arg(ap, char *);
			sz = va_arg(ap, size_t);
			r = scan_atom(str, sz) != NULL;
			break;
		case 's':
			str = va_arg(ap, char *);
			sz = va_arg(ap, size_t);
			r = scan_str(str, sz) != NULL;
			break;
		case 'A':
			strptr = va_arg(ap, char **);
			*strptr = scan_atom(NULL, 0);
			r = *strptr != NULL;
			break;
		case 'S':
			strptr = va_arg(ap, char **);
			*strptr = scan_str(NULL, 0);
			r = *strptr != NULL;
			break;
		case 'd':
			r = scanf("%d", va_arg(ap, int *));
			break;
		case 'f':
			r = scanf("%lf", va_arg(ap, double *));
			break;
		case '(':
		case ')':
			if (getchar() != c) {
				va_end(ap);
				return n;
			}
			r = 1;
			break;
		case ' ':
		case '\n':
		case '\t':
			/* no-op */
			continue;
		}
		if (r == 0) {
			/* error */
			break;
		}
		n += r;
	}
	va_end(ap);
	return n;
}

Copyright (c) 2020 Drew DeVault

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
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of the Software, and to permit persons to whom the Software is furnished to do
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