src: Move JSON parsing code to its own file

The `json_ui.cc` file contained both data-parsing and UI-related
code. This commit moves the JSON parsing code to its own `json.cc`
file, to separate concerns, make compilation faster when changes are
made to either UI or parsing code, and make the parsing code more
accessible to fuzzers.

The signature of the following function:

```
auto parse_json(StringView json);
```

was changed to:

```
JsonResult parse_json(StringView json);
```

to avoid `auto` deduction issues at compile-time.

1 files changed, 175 insertions, 0 deletions

diff --git a/src/json.cc b/src/json.cc
new file mode 100644
index 00000000..dcb4005a
--- /dev/null
+++ b/src/json.cc
@@ -0,0 +1,175 @@
+#include "json.hh"
+
+#include "exception.hh"
+#include "string_utils.hh"
+#include "unit_tests.hh"
+#include "utils.hh"
+
+#include <cstdio>
+
+namespace Kakoune
+{
+
+String to_json(int i) { return to_string(i); }
+String to_json(bool b) { return b ? "true" : "false"; }
+String to_json(StringView str)
+{
+    String res;
+    res.reserve(str.length() + 4);
+    res += '"';
+    for (auto it = str.begin(), end = str.end(); it != end; )
+    {
+        auto next = std::find_if(it, end, [](char c) {
+            return c == '\\' or c == '"' or (c >= 0 and c <= 0x1F);
+        });
+
+        res += StringView{it, next};
+        if (next == end)
+            break;
+
+        char buf[7] = {'\\', *next, 0};
+        if (*next >= 0 and *next <= 0x1F)
+            sprintf(buf, "\\u%04x", *next);
+
+        res += buf;
+        it = next+1;
+    }
+    res += '"';
+    return res;
+}
+
+static bool is_digit(char c) { return c >= '0' and c <= '9'; }
+
+JsonResult parse_json(const char* pos, const char* end)
+{
+    if (not skip_while(pos, end, is_blank))
+        return {};
+
+    if (is_digit(*pos) or *pos == '-')
+    {
+        auto digit_end = pos + 1;
+        skip_while(digit_end, end, is_digit);
+        return { Value{str_to_int({pos, digit_end})}, digit_end };
+    }
+    if (end - pos > 4 and StringView{pos, pos+4} == "true")
+        return { Value{true}, pos+4 };
+    if (end - pos > 5 and StringView{pos, pos+5} == "false")
+        return { Value{false}, pos+5 };
+    if (*pos == '"')
+    {
+        String value;
+        bool escaped = false;
+        ++pos;
+        for (auto string_end = pos; string_end != end; ++string_end)
+        {
+            if (escaped)
+            {
+                escaped = false;
+                value += StringView{pos, string_end};
+                value.back() = *string_end;
+                pos = string_end+1;
+                continue;
+            }
+            if (*string_end == '\\')
+                escaped = true;
+            if (*string_end == '"')
+            {
+                value += StringView{pos, string_end};
+                return {std::move(value), string_end+1};
+            }
+        }
+        return {};
+    }
+    if (*pos == '[')
+    {
+        JsonArray array;
+        if (++pos == end)
+            throw runtime_error("unable to parse array");
+        if (*pos == ']')
+            return {std::move(array), pos+1};
+
+        while (true)
+        {
+            auto [element, new_pos] = parse_json(pos, end);
+            if (not element)
+                return {};
+            pos = new_pos;
+            array.push_back(std::move(element));
+            if (not skip_while(pos, end, is_blank))
+                return {};
+
+            if (*pos == ',')
+                ++pos;
+            else if (*pos == ']')
+                return {std::move(array), pos+1};
+            else
+                throw runtime_error("unable to parse array, expected ',' or ']'");
+        }
+    }
+    if (*pos == '{')
+    {
+        if (++pos == end)
+            throw runtime_error("unable to parse object");
+        JsonObject object;
+        if (*pos == '}')
+            return {std::move(object), pos+1};
+
+        while (true)
+        {
+            auto [name_value, name_end] = parse_json(pos, end);
+            if (not name_value)
+                return {};
+            pos = name_end;
+            String& name = name_value.as<String>();
+            if (not skip_while(pos, end, is_blank))
+                return {};
+            if (*pos++ != ':')
+                throw runtime_error("expected :");
+
+            auto [element, element_end] = parse_json(pos, end);
+            if (not element)
+                return {};
+            pos = element_end;
+            object.insert({ std::move(name), std::move(element) });
+            if (not skip_while(pos, end, is_blank))
+                return {};
+
+            if (*pos == ',')
+                ++pos;
+            else if (*pos == '}')
+                return {std::move(object), pos+1};
+            else
+                throw runtime_error("unable to parse object, expected ',' or '}'");
+        }
+    }
+    throw runtime_error("unable to parse json");
+}
+
+JsonResult parse_json(StringView json) { return parse_json(json.begin(), json.end()); }
+
+UnitTest test_json_parser{[]()
+{
+    {
+        auto value = parse_json(R"({ "jsonrpc": "2.0", "method": "keys", "params": [ "b", "l", "a", "h" ] })").value;
+        kak_assert(value);
+    }
+
+    {
+        auto value = parse_json("[10,20]").value;
+        kak_assert(value and value.is_a<JsonArray>());
+        kak_assert(value.as<JsonArray>().at(1).as<int>() == 20);
+    }
+
+    {
+        auto value = parse_json("-1").value;
+        kak_assert(value.as<int>() == -1);
+    }
+
+    {
+        auto value = parse_json("{}").value;
+        kak_assert(value and value.is_a<JsonObject>());
+        kak_assert(value.as<JsonObject>().empty());
+    }
+}};
+
+}