works

.gitignore

@@ -1,3 +1,6 @@
 /target
 .idea
 *.iml
+
+# test data
+*.json

README.md

@@ -0,0 +1,7 @@
+Formats json.
+
+Will maybe even be fast in the future, idk
+
+would be amazing if it even works.
+
+note: does not actually parse the json to a parse tree or something, it just formats it

src/lib.rs

@@ -0,0 +1,149 @@
+pub fn format_json(json: &str, indent_str: &str) -> String {
+    let mut out = String::with_capacity(json.len()); // at least as big as the input
+
+    let mut escaped = false;
+    let mut in_string = false;
+    let mut indent_level = 0;
+    let mut newline_requested = false; // invalidated if next character is ] or }
+
+    for char in json.chars() {
+        if in_string {
+            let mut escape_here = false;
+            match char {
+                '"' => {
+                    if !escaped {
+                        in_string = false;
+                    }
+                }
+                '\\' => {
+                    if !escaped {
+                        escape_here = true;
+                    }
+                }
+                _ => {}
+            }
+            out.push(char);
+            escaped = escape_here;
+        } else {
+            let mut auto_push = true;
+            let mut request_newline = false;
+            let old_level = indent_level;
+            match char {
+                '"' => in_string = true,
+                ' ' | '\n' | '\t' => continue,
+                '[' => {
+                    indent_level += 1;
+                    request_newline = true;
+                }
+                '{' => {
+                    indent_level += 1;
+                    request_newline = true;
+                }
+                '}' | ']' => {
+                    indent_level -= 1;
+                    if !newline_requested {
+                        // see comment above
+                        out.push('\n');
+                        indent(&mut out, indent_level, indent_str);
+                    }
+                }
+                ':' => {
+                    auto_push = false;
+                    out.push(char);
+                    out.push(' ');
+                }
+                ',' => {
+                    request_newline = true;
+                }
+                _ => {}
+            }
+            if newline_requested && char != ']' && char != '}' {
+                // newline only happens after { [ and ,
+                out.push('\n');
+                indent(&mut out, old_level, indent_str);
+            }
+
+            if auto_push {
+                out.push(char);
+            }
+
+            newline_requested = request_newline;
+        }
+    }
+
+    out
+}
+
+fn indent(buf: &mut String, level: usize, indent_str: &str) {
+    for _ in 0..level {
+        buf.push_str(indent_str);
+    }
+}
+
+#[cfg(test)]
+mod test {
+    const INDENT: &str = " ";
+    use super::*;
+
+    #[test]
+    fn echoes_primitive() {
+        let json = "1.35";
+        assert_eq!(json, format_json(json, INDENT));
+    }
+
+    #[test]
+    fn ignore_whitespace_in_string() {
+        let json = "\" hallo \"";
+        assert_eq!(json, format_json(json, INDENT));
+    }
+
+    #[test]
+    fn remove_leading_whitespace() {
+        let json = "   0";
+        let expected = "0";
+        assert_eq!(expected, format_json(json, INDENT));
+    }
+
+    #[test]
+    fn handle_escaped_strings() {
+        let json = "  \" hallo \\\" \" ";
+        let expected = "\" hallo \\\" \"";
+        assert_eq!(expected, format_json(json, INDENT));
+    }
+
+    #[test]
+    fn simple_object() {
+        let json = "{\"a\":0}";
+        let expected = "{
+ \"a\": 0
+}";
+        assert_eq!(expected, format_json(json, INDENT));
+    }
+
+    #[test]
+    fn simple_array() {
+        let json = "[1,2,null]";
+        let expected = "[
+ 1,
+ 2,
+ null
+]";
+        assert_eq!(expected, format_json(json, INDENT));
+    }
+
+    #[test]
+    fn array_of_object() {
+        let json = "[{\"a\": 0}, {}, {\"a\": null}]";
+        let expected = "[
+ {
+  \"a\": 0
+ },
+ {},
+ {
+  \"a\": null
+ }
+]";
+
+        assert_eq!(expected, format_json(json, INDENT));
+    }
+}

src/main.rs

@@ -1,3 +1,22 @@
-fn main() {
-    println!("Hello, world!");
+use jsonformat::format_json;
+use std::fs;
+use std::io;
+use std::io::Read;
+
+fn main() -> Result<(), io::Error> {
+    let filename = std::env::args().skip(1).next();
+
+    let str = match filename {
+        Some(path) => fs::read_to_string(path)?,
+        None => {
+            let mut buf = String::new();
+            let stdin = std::io::stdin();
+            stdin.lock().read_to_string(&mut buf)?;
+            buf
+        }
+    };
+
+    println!("{}", format_json(&str, "  "));
+
+    Ok(())
 }