xml FEATURE xml parser

just work in progress:
- element name parser (lyxml_get_element())
diff --git a/src/xml.c b/src/xml.c
new file mode 100644
index 0000000..c4bdb4b
--- /dev/null
+++ b/src/xml.c
@@ -0,0 +1,281 @@
+/**
+ * @file xml.c
+ * @author Radek Krejci <rkrejci@cesnet.cz>
+ * @brief Generic XML parser implementation for libyang
+ *
+ * Copyright (c) 2015 - 2018 CESNET, z.s.p.o.
+ *
+ * This source code is licensed under BSD 3-Clause License (the "License").
+ * You may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://opensource.org/licenses/BSD-3-Clause
+ */
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "libyang.h"
+#include "xml.h"
+#include "common.h"
+
+/* Macro to test if character is whitespace */
+#define is_xmlws(c) (c == 0x20 || c == 0x9 || c == 0xa || c == 0xd)
+
+/* Macro to test if character is allowed to be a first character of an qualified identifier */
+#define is_xmlqnamestartchar(c) ((c >= 'a' && c <= 'z') || c == '_' || \
+        (c >= 'A' && c <= 'Z') || /* c == ':' || */ \
+        (c >= 0x370 && c <= 0x1fff && c != 0x37e ) || \
+        (c >= 0xc0 && c <= 0x2ff && c != 0xd7 && c != 0xf7) || c == 0x200c || \
+        c == 0x200d || (c >= 0x2070 && c <= 0x218f) || \
+        (c >= 0x2c00 && c <= 0x2fef) || (c >= 0x3001 && c <= 0xd7ff) || \
+        (c >= 0xf900 && c <= 0xfdcf) || (c >= 0xfdf0 && c <= 0xfffd) || \
+        (c >= 0x10000 && c <= 0xeffff))
+
+/* Macro to test if character is allowed to be used in an qualified identifier */
+#define is_xmlqnamechar(c) ((c >= 'a' && c <= 'z') || c == '_' || c == '-' || \
+        (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || /* c == ':' || */ \
+        c == '.' || c == 0xb7 || (c >= 0x370 && c <= 0x1fff && c != 0x37e ) ||\
+        (c >= 0xc0 && c <= 0x2ff && c != 0xd7 && c != 0xf7) || c == 0x200c || \
+        c == 0x200d || (c >= 0x300 && c <= 0x36f) || \
+        (c >= 0x2070 && c <= 0x218f) || (c >= 0x2030f && c <= 0x2040) || \
+        (c >= 0x2c00 && c <= 0x2fef) || (c >= 0x3001 && c <= 0xd7ff) || \
+        (c >= 0xf900 && c <= 0xfdcf) || (c >= 0xfdf0 && c <= 0xfffd) || \
+        (c >= 0x10000 && c <= 0xeffff))
+
+/* Move input p by s characters, if EOF log with lyxml_context c */
+#define move_input(c,p,s) p += s; LY_CHECK_ERR_RET(!p[0], LOGVAL(c->ctx, LY_VLOG_LINE, &c->line, LY_VCODE_EOF), LY_EVALID)
+
+/* Ignore whitespaces in the input string p, if EOF log with lyxml_context c */
+#define ign_xmlws(c,p) while (is_xmlws(*(p))) {if (*(p) == '\n') {++c->line;} ++p;}
+
+static const char *
+ign_todelim(register const char *input, const char *delim, size_t delim_len, size_t *newlines)
+{
+    size_t i;
+    register const char *a, *b;
+
+    (*newlines) = 0;
+    for ( ; *input; ++input) {
+        if (*input != *delim) {
+            if (*input == '\n') {
+                ++(*newlines);
+            }
+            continue;
+        }
+        a = input;
+        b = delim;
+        for (i = 0; i < delim_len; ++i) {
+            if (*a++ != *b++) {
+                break;
+            }
+        }
+        if (i == delim_len) {
+            return input;
+        }
+    }
+    return NULL;
+}
+
+static LY_ERR
+lyxml_getutf8(const char **input, unsigned int *utf8_char, size_t *bytes_read)
+{
+    unsigned int c, len;
+    int aux;
+    int i;
+
+    c = (*input)[0];
+    LY_CHECK_RET(!c, LY_EINVAL);
+
+    /* process character byte(s) */
+    if ((c & 0xf8) == 0xf0) {
+        /* four bytes character */
+        len = 4;
+
+        c &= 0x07;
+        for (i = 1; i <= 3; i++) {
+            aux = (*input)[i];
+            if ((aux & 0xc0) != 0x80) {
+                return LY_EINVAL;
+            }
+
+            c = (c << 6) | (aux & 0x3f);
+        }
+
+        if (c < 0x1000 || c > 0x10ffff) {
+            return LY_EINVAL;
+        }
+    } else if ((c & 0xf0) == 0xe0) {
+        /* three bytes character */
+        len = 3;
+
+        c &= 0x0f;
+        for (i = 1; i <= 2; i++) {
+            aux = (*input)[i];
+            if ((aux & 0xc0) != 0x80) {
+                return LY_EINVAL;
+            }
+
+            c = (c << 6) | (aux & 0x3f);
+        }
+
+        if (c < 0x800 || (c > 0xd7ff && c < 0xe000) || c > 0xfffd) {
+            return LY_EINVAL;
+        }
+    } else if ((c & 0xe0) == 0xc0) {
+        /* two bytes character */
+        len = 2;
+
+        aux = (*input)[1];
+        if ((aux & 0xc0) != 0x80) {
+            return LY_EINVAL;
+        }
+        c = ((c & 0x1f) << 6) | (aux & 0x3f);
+
+        if (c < 0x80) {
+            return LY_EINVAL;
+        }
+    } else if (!(c & 0x80)) {
+        /* one byte character */
+        len = 1;
+
+        if (c < 0x20 && c != 0x9 && c != 0xa && c != 0xd) {
+            return LY_EINVAL;
+        }
+    } else {
+        return LY_EINVAL;
+    }
+
+    (*utf8_char) = c;
+    (*input) += len;
+    if (bytes_read) {
+        (*bytes_read) = len;
+    }
+    return LY_SUCCESS;
+}
+
+LY_ERR
+lyxml_check_qname(struct lyxml_context *context, const char **input, unsigned int *term_char, size_t *term_char_len)
+{
+    unsigned int c;
+    const char *id = (*input);
+    LY_ERR rc;
+
+    /* check NameStartChar (minus colon) */
+    LY_CHECK_ERR_RET(lyxml_getutf8(input, &c, NULL) != LY_SUCCESS,
+                     LOGVAL(context->ctx, LY_VLOG_LINE, &context->line, LY_VCODE_INCHAR, (*input)[0]), LY_EVALID);
+    LY_CHECK_ERR_RET(!is_xmlqnamestartchar(c),
+                     LOGVAL(context->ctx, LY_VLOG_LINE, &context->line, LYVE_SYNTAX,
+                            "Identifier \"%s\" starts with invalid character.", id),
+                     LY_EVALID);
+
+    /* check rest of the identifier */
+    for (rc = lyxml_getutf8(input, &c, term_char_len);
+         rc == LY_SUCCESS && is_xmlqnamechar(c);
+         rc = lyxml_getutf8(input, &c, term_char_len));
+    LY_CHECK_ERR_RET(rc != LY_SUCCESS, LOGVAL(context->ctx, LY_VLOG_LINE, &context->line, LY_VCODE_INCHAR, (*input)[0]), LY_EVALID);
+
+    (*term_char) = c;
+    return LY_SUCCESS;
+}
+
+LY_ERR
+lyxml_get_element(struct lyxml_context *context, const char **input, int UNUSED(options),
+                  const char **prefix, size_t *prefix_len, const char **name, size_t *name_len)
+{
+    struct ly_ctx *ctx = context->ctx; /* shortcut */
+    const char *in = (*input);
+    const char *endtag;
+    const char *sectname;
+    const char *id;
+    size_t endtag_len, newlines;
+    bool loop = true;
+    unsigned int c;
+    LY_ERR rc;
+    uint32_t x;
+
+    /* initialize output variables */
+    (*prefix) = (*name) = NULL;
+    (*prefix_len) = (*name_len) = 0;
+
+    while (loop) {
+        ign_xmlws(context, in);
+
+        if (in[0] == '\0') {
+            /* EOF */
+            goto success;
+        } else if (in[0] != '<') {
+            return LY_EINVAL;
+        }
+        move_input(context, in, 1);
+
+        if (in[0] == '!') {
+            move_input(context, in, 1);
+            /* sections to ignore */
+            if (!strncmp(in, "--", 2)) {
+                /* comment */
+                move_input(context, in, 2);
+                sectname = "Comment";
+                endtag = "-->";
+                endtag_len = 3;
+            } else if (!strncmp(in, "[CDATA[", 7)) {
+                /* CDATA section */
+                move_input(context, in, 7);
+                sectname = "CData";
+                endtag = "]]>";
+                endtag_len = 3;
+            } else if (!strncmp(in, "DOCTYPE", 7)) {
+                /* Document type declaration - not supported */
+                LOGVAL(ctx, LY_VLOG_LINE, &context->line, LY_VCODE_NSUPP, "Document Type Declaration");
+                return LY_EVALID;
+            }
+            in = ign_todelim(in, endtag, endtag_len, &newlines);
+            LY_CHECK_ERR_RET(!in, LOGVAL(ctx, LY_VLOG_LINE, &context->line, LY_VCODE_NTERM, sectname), LY_EVALID);
+            context->line += newlines;
+            in += endtag_len;
+        } else if (in[0] == '?') {
+            in = ign_todelim(in, "?>", 2, &newlines);
+            LY_CHECK_ERR_RET(!in, LOGVAL(ctx, LY_VLOG_LINE, &context->line, LY_VCODE_NTERM, "Declaration"), LY_EVALID);
+            context->line += newlines;
+            in += 2;
+        } else {
+            /* element */
+            ign_xmlws(context, in);
+            LY_CHECK_ERR_RET(!in[0], LOGVAL(ctx, LY_VLOG_LINE, &context->line, LY_VCODE_EOF), LY_EVALID);
+
+            /* remember the identifier start before checking its format */
+            id = in;
+            rc = lyxml_check_qname(context, &in, &c, &endtag_len);
+            LY_CHECK_RET(rc);
+            if (c == ':') {
+                /* we have prefixed identifier */
+                endtag = in - endtag_len;
+
+                rc = lyxml_check_qname(context, &in, &c, &endtag_len);
+                LY_CHECK_RET(rc);
+
+                (*prefix) = id;
+                (*prefix_len) = endtag - id;
+                id = endtag + 1;
+            }
+            if (!is_xmlws(c) && c != '/' && c != '>') {
+                in = in - endtag_len;
+                x = 0;
+                memcpy(&x, in, endtag_len);
+                LOGVAL(ctx, LY_VLOG_LINE, &context->line, LY_VCODE_INCHAR, x);
+                return LY_EVALID;
+            }
+            in = in - endtag_len;
+            (*name) = id;
+            (*name_len) = in - id;
+
+            loop = false;
+        }
+    }
+
+success:
+    /* move caller's input */
+    (*input) = in;
+    return LY_SUCCESS;
+}
+