Add support for parsing variables

This adds a reader flag and serdi option for extending a syntax with support
for SPARQL-like variables, for storing things like patterns or simple queries.

7 files changed, 97 insertions, 5 deletions

diff --git a/src/env.c b/src/env.c
index 55901e26..a11abc70 100644
--- a/src/env.c
+++ b/src/env.c
@@ -355,6 +355,7 @@ serd_env_expand(const SerdEnv* env, const SerdNode* node)
     case SERD_CURIE:
       return expand_curie(env, node);
     case SERD_BLANK:
+    case SERD_VARIABLE:
       return serd_node_copy(node);
     }
   }
diff --git a/src/n3.c b/src/n3.c
index c9f2dd6b..1ae98a56 100644
--- a/src/n3.c
+++ b/src/n3.c
@@ -929,10 +929,53 @@ read_literal(SerdReader* reader, SerdNode** dest, bool* ate_dot)
 }
 
 static SerdStatus
+read_VARNAME(SerdReader* reader, SerdNode** dest)
+{
+  // Simplified from SPARQL: VARNAME ::= (PN_CHARS_U | [0-9])+
+  SerdNode*  n  = *dest;
+  SerdStatus st = SERD_SUCCESS;
+  int        c  = 0;
+  peek_byte(reader);
+  while ((c = peek_byte(reader))) {
+    if (is_digit(c) || c == '_') {
+      push_byte(reader, n, eat_byte_safe(reader, c));
+    } else if ((st = read_PN_CHARS(reader, n))) {
+      st = st > SERD_FAILURE ? st : SERD_SUCCESS;
+      break;
+    }
+  }
+
+  return st;
+}
+
+static SerdStatus
+read_Var(SerdReader* reader, SerdNode** dest)
+{
+  if (!(reader->flags & SERD_READ_VARIABLES)) {
+    return r_err(
+      reader, SERD_ERR_BAD_SYNTAX, "syntax does not support variables\n");
+  }
+
+  if (!(*dest = push_node(reader, SERD_VARIABLE, "", 0))) {
+    return SERD_ERR_OVERFLOW;
+  }
+
+  assert(peek_byte(reader) == '$' || peek_byte(reader) == '?');
+  serd_byte_source_advance(reader->source);
+
+  return read_VARNAME(reader, dest);
+}
+
+static SerdStatus
 read_verb(SerdReader* reader, SerdNode** dest)
 {
   const size_t orig_stack_size = reader->stack.size;
-  if (peek_byte(reader) == '<') {
+
+  switch (peek_byte(reader)) {
+  case '$':
+  case '?':
+    return read_Var(reader, dest);
+  case '<':
     return read_IRIREF(reader, dest);
   }
 
@@ -1110,6 +1153,12 @@ read_object(SerdReader* reader, ReadContext* ctx, bool emit, bool* ate_dot)
     case '<':
     case '_':
       break;
+    case '$':
+    case '?':
+      if (reader->flags & SERD_READ_VARIABLES) {
+        break;
+      }
+      break;
     default:
       return r_err(reader, SERD_ERR_BAD_SYNTAX, "expected: ':', '<', or '_'\n");
     }
@@ -1119,6 +1168,10 @@ read_object(SerdReader* reader, ReadContext* ctx, bool emit, bool* ate_dot)
   case EOF:
   case ')':
     return r_err(reader, SERD_ERR_BAD_SYNTAX, "expected object\n");
+  case '$':
+  case '?':
+    ret = read_Var(reader, &o);
+    break;
   case '[':
     simple = false;
     ret    = read_anon(reader, *ctx, false, &o);
@@ -1341,6 +1394,10 @@ read_subject(SerdReader* reader, ReadContext ctx, SerdNode** dest, int* s_type)
   SerdStatus st      = SERD_SUCCESS;
   bool       ate_dot = false;
   switch ((*s_type = peek_byte(reader))) {
+  case '$':
+  case '?':
+    st = read_Var(reader, dest);
+    break;
   case '[':
     st = read_anon(reader, ctx, true, dest);
     break;
@@ -1598,6 +1655,7 @@ read_n3_statement(SerdReader* reader)
       if (s_type == '(' || (s_type == '[' && !*ctx.flags)) {
         return r_err(reader, SERD_ERR_BAD_SYNTAX, "invalid graph name\n");
       }
+
       ctx.graph   = ctx.subject;
       ctx.subject = NULL;
       TRY(st, read_wrappedGraph(reader, &ctx));
@@ -1613,6 +1671,7 @@ read_n3_statement(SerdReader* reader)
       }
 
       return st > SERD_FAILURE ? st : SERD_ERR_BAD_SYNTAX;
+
     } else if (!ate_dot) {
       read_ws_star(reader);
       st = (eat_byte_check(reader, '.') == '.') ? SERD_SUCCESS
@@ -1671,10 +1730,22 @@ read_nquadsDoc(SerdReader* reader)
       return SERD_ERR_BAD_SYNTAX;
     }
 
-    // subject predicate object
     if ((st = read_subject(reader, ctx, &ctx.subject, &s_type)) ||
-        !read_ws_star(reader) || (st = read_IRIREF(reader, &ctx.predicate)) ||
-        !read_ws_star(reader) ||
+        !read_ws_star(reader)) {
+      return st;
+    }
+
+    switch (peek_byte(reader)) {
+    case '$':
+    case '?':
+      st = read_Var(reader, &ctx.predicate);
+      break;
+    case '<':
+      st = read_IRIREF(reader, &ctx.predicate);
+      break;
+    }
+
+    if (st || !read_ws_star(reader) ||
         (st = read_object(reader, &ctx, false, &ate_dot))) {
       return st;
     }
diff --git a/src/node.c b/src/node.c
index 14f6490b..4c76ed9b 100644
--- a/src/node.c
+++ b/src/node.c
@@ -165,7 +165,8 @@ serd_node_zero_pad(SerdNode* node)
 SerdNode*
 serd_new_simple_node(SerdNodeType type, const SerdStringView str)
 {
-  if (type != SERD_BLANK && type != SERD_CURIE && type != SERD_URI) {
+  if (type != SERD_BLANK && type != SERD_CURIE && type != SERD_URI &&
+      type != SERD_VARIABLE) {
     return NULL;
   }
 
diff --git a/src/reader.c b/src/reader.c
index 1c9469af..4c4bffa5 100644
--- a/src/reader.c
+++ b/src/reader.c
@@ -172,6 +172,7 @@ serd_reader_new(SerdWorld* const      world,
   me->sink    = sink;
   me->stack   = serd_stack_new(stack_size, sizeof(SerdNode));
   me->syntax  = syntax;
+  me->flags   = flags;
   me->next_id = 1;
   me->strict  = !(flags & SERD_READ_LAX);
 
diff --git a/src/reader.h b/src/reader.h
index 62feeee6..9bdbf72f 100644
--- a/src/reader.h
+++ b/src/reader.h
@@ -60,6 +60,7 @@ struct SerdReaderImpl {
   SerdByteSource* source;
   SerdStack       stack;
   SerdSyntax      syntax;
+  SerdReaderFlags flags;
   unsigned        next_id;
   uint8_t*        buf;
   char*           bprefix;
diff --git a/src/serdi.c b/src/serdi.c
index 1925cd88..2801a2da 100644
--- a/src/serdi.c
+++ b/src/serdi.c
@@ -72,6 +72,7 @@ print_usage(const char* name, bool error)
   fprintf(os, "  -s INPUT     Parse INPUT as string.\n");
   fprintf(os, "  -t           Write terser output without newlines.\n");
   fprintf(os, "  -v           Display version information and exit.\n");
+  fprintf(os, "  -x           Support parsing variable nodes like `?x'.\n");
   return error ? 1 : 0;
 }
 
@@ -246,6 +247,8 @@ main(int argc, char** argv)
         return missing_arg(argv[0], 'w');
       }
       out_filename = argv[a];
+    } else if (argv[a][1] == 'x') {
+      reader_flags |= SERD_READ_VARIABLES;
     } else {
       SERDI_ERRORF("invalid option -- '%s'\n", argv[a] + 1);
       return print_usage(argv[0], true);
diff --git a/src/writer.c b/src/writer.c
index b1404a91..fbc9dd5d 100644
--- a/src/writer.c
+++ b/src/writer.c
@@ -881,6 +881,18 @@ write_blank(SerdWriter* const        writer,
 }
 
 SERD_WARN_UNUSED_RESULT static SerdStatus
+write_variable(SerdWriter* const writer, const SerdNode* node)
+{
+  SerdStatus st = SERD_SUCCESS;
+
+  TRY(st, esink("?", 1, writer));
+  TRY(st, esink(serd_node_string(node), node->n_bytes, writer));
+
+  writer->last_sep = SEP_NONE;
+  return st;
+}
+
+SERD_WARN_UNUSED_RESULT static SerdStatus
 write_node(SerdWriter*        writer,
            const SerdNode*    node,
            const SerdField    field,
@@ -895,6 +907,8 @@ write_node(SerdWriter*        writer,
     return write_curie(writer, node, field, flags);
   case SERD_BLANK:
     return write_blank(writer, node, field, flags);
+  case SERD_VARIABLE:
+    return write_variable(writer, node);
   }
 
   return SERD_ERR_INTERNAL;