nix path-info: Add --json flag

Also, factor out JSON generation from value-to-json.{cc,hh}, and
support producing indented JSON.

src/libutil/json.cc

@@ -0,0 +1,171 @@
+#include "json.hh"
+
+#include <iomanip>
+#include <cstring>
+
+namespace nix {
+
+void toJSON(std::ostream & str, const char * start, const char * end)
+{
+    str << '"';
+    for (auto i = start; i != end; i++)
+        if (*i == '\"' || *i == '\\') str << '\\' << *i;
+        else if (*i == '\n') str << "\\n";
+        else if (*i == '\r') str << "\\r";
+        else if (*i == '\t') str << "\\t";
+        else if (*i >= 0 && *i < 32)
+            str << "\\u" << std::setfill('0') << std::setw(4) << std::hex << (uint16_t) *i << std::dec;
+        else str << *i;
+    str << '"';
+}
+
+void toJSON(std::ostream & str, const std::string & s)
+{
+    toJSON(str, s.c_str(), s.c_str() + s.size());
+}
+
+void toJSON(std::ostream & str, const char * s)
+{
+    if (!s) str << "null"; else toJSON(str, s, s + strlen(s));
+}
+
+void toJSON(std::ostream & str, unsigned long n)
+{
+    str << n;
+}
+
+void toJSON(std::ostream & str, long n)
+{
+    str << n;
+}
+
+void toJSON(std::ostream & str, double f)
+{
+    str << f;
+}
+
+void toJSON(std::ostream & str, bool b)
+{
+    str << (b ? "true" : "false");
+}
+
+JSONWriter::JSONWriter(std::ostream & str, bool indent)
+    : state(new JSONState(str, indent))
+{
+    state->stack.push_back(this);
+}
+
+JSONWriter::JSONWriter(JSONState * state)
+    : state(state)
+{
+    state->stack.push_back(this);
+}
+
+JSONWriter::~JSONWriter()
+{
+    assertActive();
+    state->stack.pop_back();
+    if (state->stack.empty()) delete state;
+}
+
+void JSONWriter::comma()
+{
+    assertActive();
+    if (first) {
+        first = false;
+    } else {
+        state->str << ',';
+    }
+    if (state->indent) indent();
+}
+
+void JSONWriter::indent()
+{
+    state->str << '\n' << std::string(state->depth * 2, ' ');
+}
+
+void JSONList::open()
+{
+    state->depth++;
+    state->str << '[';
+}
+
+JSONList::~JSONList()
+{
+    state->depth--;
+    if (state->indent && !first) indent();
+    state->str << "]";
+}
+
+JSONList JSONList::list()
+{
+    comma();
+    return JSONList(state);
+}
+
+JSONObject JSONList::object()
+{
+    comma();
+    return JSONObject(state);
+}
+
+JSONPlaceholder JSONList::placeholder()
+{
+    comma();
+    return JSONPlaceholder(state);
+}
+
+void JSONObject::open()
+{
+    state->depth++;
+    state->str << '{';
+}
+
+JSONObject::~JSONObject()
+{
+    state->depth--;
+    if (state->indent && !first) indent();
+    state->str << "}";
+}
+
+void JSONObject::attr(const std::string & s)
+{
+    comma();
+    toJSON(state->str, s);
+    state->str << ':';
+    if (state->indent) state->str << ' ';
+}
+
+JSONList JSONObject::list(const std::string & name)
+{
+    attr(name);
+    return JSONList(state);
+}
+
+JSONObject JSONObject::object(const std::string & name)
+{
+    attr(name);
+    return JSONObject(state);
+}
+
+JSONPlaceholder JSONObject::placeholder(const std::string & name)
+{
+    attr(name);
+    return JSONPlaceholder(state);
+}
+
+JSONList JSONPlaceholder::list()
+{
+    assertValid();
+    first = false;
+    return JSONList(state);
+}
+
+JSONObject JSONPlaceholder::object()
+{
+    assertValid();
+    first = false;
+    return JSONObject(state);
+}
+
+}