Add SerdField to public API

A statement field enum was previously only used by the writer internally, but
will be used in the public API (without a sentinel value) in following commits.

2 files changed, 55 insertions, 47 deletions

diff --git a/include/serd/serd.h b/include/serd/serd.h
index 3be32a65..820af22c 100644
--- a/include/serd/serd.h
+++ b/include/serd/serd.h
@@ -827,6 +827,20 @@ serd_node_equals(const SerdNode* SERD_NULLABLE a,
 
 /**
    @}
+   @defgroup serd_statement Statement
+   @{
+*/
+
+/// Index of a node in a statement
+typedef enum {
+  SERD_SUBJECT   = 0, ///< Subject
+  SERD_PREDICATE = 1, ///< Predicate ("key")
+  SERD_OBJECT    = 2, ///< Object ("value")
+  SERD_GRAPH     = 3, ///< Graph ("context")
+} SerdField;
+
+/**
+   @}
    @defgroup serd_world World
    @{
 */
diff --git a/src/writer.c b/src/writer.c
index 6def5f73..8bbd6519 100644
--- a/src/writer.c
+++ b/src/writer.c
@@ -33,14 +33,6 @@
 #include <stdlib.h>
 #include <string.h>
 
-typedef enum {
-  FIELD_NONE,
-  FIELD_SUBJECT,
-  FIELD_PREDICATE,
-  FIELD_OBJECT,
-  FIELD_GRAPH
-} Field;
-
 typedef struct {
   SerdNode* graph;
   SerdNode* subject;
@@ -122,7 +114,7 @@ serd_writer_set_prefix(SerdWriter*     writer,
 static bool
 write_node(SerdWriter*        writer,
            const SerdNode*    node,
-           Field              field,
+           SerdField          field,
            SerdStatementFlags flags);
 
 static bool
@@ -147,14 +139,14 @@ anon_stack_top(SerdWriter* const writer)
 }
 
 static SerdNode*
-ctx(SerdWriter* writer, const Field field)
+ctx(SerdWriter* writer, const SerdField field)
 {
   SerdNode* node = NULL;
-  if (field == FIELD_SUBJECT) {
+  if (field == SERD_SUBJECT) {
     node = writer->context.subject;
-  } else if (field == FIELD_PREDICATE) {
+  } else if (field == SERD_PREDICATE) {
     node = writer->context.predicate;
-  } else if (field == FIELD_GRAPH) {
+  } else if (field == SERD_GRAPH) {
     node = writer->context.graph;
   }
 
@@ -494,11 +486,13 @@ free_context(const WriteContext* const ctx)
 }
 
 static bool
-is_inline_start(const SerdWriter* writer, Field field, SerdStatementFlags flags)
+is_inline_start(const SerdWriter*  writer,
+                SerdField          field,
+                SerdStatementFlags flags)
 {
   return (supports_abbrev(writer) &&
-          ((field == FIELD_SUBJECT && (flags & SERD_ANON_S_BEGIN)) ||
-           (field == FIELD_OBJECT && (flags & SERD_ANON_O_BEGIN))));
+          ((field == SERD_SUBJECT && (flags & SERD_ANON_S_BEGIN)) ||
+           (field == SERD_OBJECT && (flags & SERD_ANON_O_BEGIN))));
 }
 
 static bool
@@ -545,7 +539,7 @@ write_literal(SerdWriter* const        writer,
     sink(serd_node_string(lang), lang->length, writer);
   } else if (type_uri) {
     sink("^^", 2, writer);
-    return write_node(writer, datatype, FIELD_NONE, flags);
+    return write_node(writer, datatype, (SerdField)-1, flags);
   }
   return true;
 }
@@ -565,9 +559,9 @@ is_name(const char* buf, const size_t len)
 }
 
 static bool
-write_uri_node(SerdWriter* const writer,
-               const SerdNode*   node,
-               const Field       field)
+write_uri_node(SerdWriter* const     writer,
+               const SerdNode* const node,
+               const SerdField       field)
 {
   const SerdNode* prefix = NULL;
   SerdStringView  suffix = {NULL, 0};
@@ -575,7 +569,7 @@ write_uri_node(SerdWriter* const writer,
   const char* node_str   = serd_node_string(node);
   const bool  has_scheme = serd_uri_string_has_scheme(node_str);
   if (supports_abbrev(writer)) {
-    if (field == FIELD_PREDICATE && !strcmp(node_str, NS_RDF "type")) {
+    if (field == SERD_PREDICATE && !strcmp(node_str, NS_RDF "type")) {
       return sink("a", 1, writer) == 1;
     }
 
@@ -657,7 +651,7 @@ write_curie(SerdWriter* const writer, const SerdNode* const node)
 static bool
 write_blank(SerdWriter* const        writer,
             const SerdNode*          node,
-            const Field              field,
+            const SerdField          field,
             const SerdStatementFlags flags)
 {
   const char* node_str = serd_node_string(node);
@@ -667,7 +661,7 @@ write_blank(SerdWriter* const        writer,
       return write_sep(writer, SEP_ANON_BEGIN);
     }
 
-    if (field == FIELD_SUBJECT && (flags & SERD_LIST_S_BEGIN)) {
+    if (field == SERD_SUBJECT && (flags & SERD_LIST_S_BEGIN)) {
       assert(writer->list_depth == 0);
       serd_node_set(&writer->list_subj, node);
       ++writer->list_depth;
@@ -675,14 +669,14 @@ write_blank(SerdWriter* const        writer,
       return write_sep(writer, SEP_LIST_BEGIN);
     }
 
-    if (field == FIELD_OBJECT && (flags & SERD_LIST_O_BEGIN)) {
+    if (field == SERD_OBJECT && (flags & SERD_LIST_O_BEGIN)) {
       ++writer->indent;
       ++writer->list_depth;
       return write_sep(writer, SEP_LIST_BEGIN);
     }
 
-    if ((field == FIELD_SUBJECT && (flags & SERD_EMPTY_S)) ||
-        (field == FIELD_OBJECT && (flags & SERD_EMPTY_O))) {
+    if ((field == SERD_SUBJECT && (flags & SERD_EMPTY_S)) ||
+        (field == SERD_OBJECT && (flags & SERD_EMPTY_O))) {
       return sink("[]", 2, writer) == 2;
     }
   }
@@ -703,7 +697,7 @@ write_blank(SerdWriter* const        writer,
 static bool
 write_node(SerdWriter* const        writer,
            const SerdNode* const    node,
-           const Field              field,
+           const SerdField          field,
            const SerdStatementFlags flags)
 {
   bool ret = false;
@@ -734,7 +728,7 @@ is_resource(const SerdNode* node)
 static void
 write_pred(SerdWriter* writer, SerdStatementFlags flags, const SerdNode* pred)
 {
-  write_node(writer, pred, FIELD_PREDICATE, flags);
+  write_node(writer, pred, SERD_PREDICATE, flags);
   write_sep(writer, SEP_P_O);
   serd_node_set(&writer->context.predicate, pred);
 }
@@ -753,7 +747,7 @@ write_list_obj(SerdWriter* const        writer,
 
   if (!strcmp(serd_node_string(predicate), NS_RDF "first")) {
     write_sep(writer, SEP_LIST_SEP);
-    write_node(writer, object, FIELD_OBJECT, flags);
+    write_node(writer, object, SERD_OBJECT, flags);
   }
 
   return false;
@@ -779,32 +773,32 @@ serd_writer_write_statement(SerdWriter* const        writer,
   } while (0)
 
   if (writer->syntax == SERD_NTRIPLES || writer->syntax == SERD_NQUADS) {
-    TRY(write_node(writer, subject, FIELD_SUBJECT, flags));
+    TRY(write_node(writer, subject, SERD_SUBJECT, flags));
     sink(" ", 1, writer);
-    TRY(write_node(writer, predicate, FIELD_PREDICATE, flags));
+    TRY(write_node(writer, predicate, SERD_PREDICATE, flags));
     sink(" ", 1, writer);
-    TRY(write_node(writer, object, FIELD_OBJECT, flags));
+    TRY(write_node(writer, object, SERD_OBJECT, flags));
     if (writer->syntax == SERD_NQUADS && graph) {
       sink(" ", 1, writer);
-      TRY(write_node(writer, graph, FIELD_GRAPH, flags));
+      TRY(write_node(writer, graph, SERD_GRAPH, flags));
     }
     sink(" .\n", 3, writer);
     return SERD_SUCCESS;
   }
 
   if ((graph && !serd_node_equals(graph, writer->context.graph)) ||
-      (!graph && ctx(writer, FIELD_GRAPH))) {
+      (!graph && ctx(writer, SERD_GRAPH))) {
     writer->indent = 0;
-    if (ctx(writer, FIELD_SUBJECT)) {
+    if (ctx(writer, SERD_SUBJECT)) {
       write_sep(writer, SEP_END_S);
     }
-    if (ctx(writer, FIELD_GRAPH)) {
+    if (ctx(writer, SERD_GRAPH)) {
       write_sep(writer, SEP_GRAPH_END);
     }
 
     reset_context(writer, true);
     if (graph) {
-      TRY(write_node(writer, graph, FIELD_GRAPH, flags));
+      TRY(write_node(writer, graph, SERD_GRAPH, flags));
       ++writer->indent;
       write_sep(writer, SEP_GRAPH_BEGIN);
       serd_node_set(&writer->context.graph, graph);
@@ -829,20 +823,20 @@ serd_writer_write_statement(SerdWriter* const        writer,
         ++writer->indent;
       }
       write_sep(writer, SEP_END_O);
-      write_node(writer, object, FIELD_OBJECT, flags);
+      write_node(writer, object, SERD_OBJECT, flags);
       if (!(flags & SERD_ANON_O_BEGIN)) {
         --writer->indent;
       }
     } else {
       // Abbreviate S
-      Sep sep = ctx(writer, FIELD_PREDICATE) ? SEP_END_P : SEP_S_P;
+      Sep sep = ctx(writer, SERD_PREDICATE) ? SEP_END_P : SEP_S_P;
       write_sep(writer, sep);
       write_pred(writer, flags, predicate);
-      write_node(writer, object, FIELD_OBJECT, flags);
+      write_node(writer, object, SERD_OBJECT, flags);
     }
   } else {
     // No abbreviation
-    if (ctx(writer, FIELD_SUBJECT)) {
+    if (ctx(writer, SERD_SUBJECT)) {
       assert(writer->indent > 0);
       --writer->indent;
       if (serd_stack_is_empty(&writer->anon_stack)) {
@@ -853,7 +847,7 @@ serd_writer_write_statement(SerdWriter* const        writer,
     }
 
     if (!(flags & SERD_ANON_CONT)) {
-      write_node(writer, subject, FIELD_SUBJECT, flags);
+      write_node(writer, subject, SERD_SUBJECT, flags);
       ++writer->indent;
       write_sep(writer, SEP_S_P);
     } else {
@@ -867,7 +861,7 @@ serd_writer_write_statement(SerdWriter* const        writer,
       write_pred(writer, flags, predicate);
     }
 
-    write_node(writer, object, FIELD_OBJECT, flags);
+    write_node(writer, object, SERD_OBJECT, flags);
   }
 
   if (flags & (SERD_ANON_S_BEGIN | SERD_ANON_O_BEGIN)) {
@@ -923,10 +917,10 @@ serd_writer_end_anon(SerdWriter* writer, const SerdNode* node)
 SerdStatus
 serd_writer_finish(SerdWriter* writer)
 {
-  if (ctx(writer, FIELD_SUBJECT)) {
+  if (ctx(writer, SERD_SUBJECT)) {
     write_sep(writer, SEP_END_S);
   }
-  if (ctx(writer, FIELD_GRAPH)) {
+  if (ctx(writer, SERD_GRAPH)) {
     write_sep(writer, SEP_GRAPH_END);
   }
   serd_byte_sink_flush(&writer->byte_sink);
@@ -994,7 +988,7 @@ serd_writer_set_base_uri(SerdWriter* writer, const SerdNode* uri)
   serd_env_set_base_uri(writer->env, serd_node_string_view(uri));
 
   if (writer->syntax == SERD_TURTLE || writer->syntax == SERD_TRIG) {
-    if (ctx(writer, FIELD_GRAPH) || ctx(writer, FIELD_SUBJECT)) {
+    if (ctx(writer, SERD_GRAPH) || ctx(writer, SERD_SUBJECT)) {
       sink(" .\n\n", 4, writer);
       reset_context(writer, true);
     }
@@ -1040,7 +1034,7 @@ serd_writer_set_prefix(SerdWriter*     writer,
   }
 
   if (writer->syntax == SERD_TURTLE || writer->syntax == SERD_TRIG) {
-    if (ctx(writer, FIELD_GRAPH) || ctx(writer, FIELD_SUBJECT)) {
+    if (ctx(writer, SERD_GRAPH) || ctx(writer, SERD_SUBJECT)) {
       sink(" .\n\n", 4, writer);
       reset_context(writer, true);
     }