Play with explicit AST

src/ast.rs

@@ -0,0 +1,140 @@
+/*
+ * Copyright 2020 VMware, Inc.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+use serde_json::Value;
+
+/// A path is a tree of selector nodes.
+///
+/// For example, the JSONPath `$.foo.bar` yields this AST:
+///
+/// ```text
+///              ^
+///             / \
+///            ^   \___ DotName("bar")
+///           / \
+/// Root ___ /   \___ DotName("foo")
+/// ```
+///
+/// A more complicated example: `$.foo[1,2]["bar"]`:
+///
+/// ```text
+///                 ^
+///                / \
+///               ^   \___ Union
+///              / \            \
+///             /   \___ Union   \___ [Field("bar")]
+///            /              \
+///           ^                \___ [Number(1), Number(2)]
+///          / \
+/// Root ___/   \___ DotName("foo")
+/// ```
+///
+/// Selectors are left associative, thus `$.foo[1,2]["bar"]` behaves
+/// like (pseudocode) `(($.foo)[1,2])["bar"]`; thus the root of the resulting
+/// tree is actually the right-most selector (the last one to be applied).
+#[derive(Debug)]
+pub enum Path {
+    Root,
+    Sel(Box<Path>, Selector),
+}
+
+#[derive(Debug)]
+pub enum Selector {
+    Union(Vec<Index>),
+    DotName(String),
+    DotWildcard,
+}
+
+#[derive(Debug)]
+pub enum Index {
+    Field(String),
+    Number(i64),
+}
+
+type Iter<'a> = Box<dyn Iterator<Item = &'a Value> + 'a>;
+
+impl Path {
+    pub fn find<'a>(&'a self, input: &'a Value) -> Iter<'a> {
+        match self {
+            Path::Root => Box::new(std::iter::once(input)),
+            Path::Sel(left, sel) => Box::new(left.find(input).flat_map(move |v| sel.find(v))),
+        }
+    }
+}
+
+impl Selector {
+    pub fn find<'a>(&'a self, input: &'a Value) -> Iter<'a> {
+        match self {
+            Selector::Union(indices) => Box::new(indices.iter().flat_map(move |i| i.get(input))),
+            Selector::DotName(name) => Box::new(input.get(name).into_iter()),
+            Selector::DotWildcard => match input {
+                Value::Object(m) => Box::new(m.values()),
+                Value::Array(a) => Box::new(a.iter()),
+                _ => Box::new(std::iter::empty()),
+            },
+        }
+    }
+}
+
+impl Index {
+    pub fn get<'a>(&self, v: &'a Value) -> Iter<'a> {
+        match self {
+            Index::Field(name) => Box::new(v.get(name).into_iter()),
+            Index::Number(num) => Box::new(v.get(abs_index(*num, v)).into_iter()),
+        }
+    }
+}
+
+fn abs_index(index: i64, node: &Value) -> usize {
+    if index >= 0 {
+        index as usize
+    } else {
+        let len = if let Value::Array(a) = node {
+            a.len() as i64
+        } else {
+            0
+        };
+        (len + index) as usize
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::parser::parse;
+    use serde_json::json;
+
+    #[test]
+    fn demo() {
+        let a1 = Path::Sel(Box::new(Path::Root), Selector::DotName("foo".to_owned()));
+        let a2 = Path::Sel(Box::new(a1), Selector::DotName("bar".to_owned()));
+        let a3 = Path::Sel(
+            Box::new(a2),
+            Selector::Union(vec![Index::Field("baz".to_owned())]),
+        );
+        let a4 = Path::Sel(Box::new(a3), Selector::Union(vec![Index::Number(4)]));
+
+        let j = json!({"foo":{"bar":{"baz":[10,20,30,40,50,60]}}});
+        println!("j: {}", j);
+
+        let v = a4.find(&j).collect::<Vec<_>>();
+        assert_eq!(v[0], 50);
+    }
+
+    #[test]
+    fn parse_demo() -> Result<(), String> {
+        let p = parse("$.foo['bar'].*[4,-1]")?;
+        println!("AST: {:?}", &p);
+        let j = json!({"foo":{"bar":{"baz":[10,20,30,40,50,60]}}});
+
+        let v = p.find(&j).collect::<Vec<_>>();
+        println!("RES: {:?}", v);
+
+        assert_eq!(v[0], 50);
+        assert_eq!(v[1], 60);
+        Ok(())
+    }
+}

src/grammar.pest

@@ -1,6 +1,6 @@
-selector = _{ SOI ~ jsonPath ~ EOI }
+selector = _{ SOI ~ rootSelector ~ jsonPath ~ EOI }
 
-jsonPath = ${ rootSelector ~ matcher* }
+jsonPath = ${ matcher* }
 rootSelector = { "$" }
 
 matcher = { dotChild | union }

src/jsonpath.rs

@@ -4,8 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+use crate::ast;
 use crate::parser;
-use crate::path::Path;
+use serde_json::Value;
 
 #[derive(Debug)]
 pub struct SyntaxError {
@@ -18,6 +19,19 @@ impl std::fmt::Display for SyntaxError {
     }
 }
 
-pub fn parse(selector: &str) -> Result<impl Path, SyntaxError> {
-    parser::parse(selector).map_err(|m| SyntaxError { message: m })
+pub enum FindError {
+    // no errors yet
+}
+
+pub fn parse(selector: &str) -> Result<Path, SyntaxError> {
+    let p = parser::parse(selector).map_err(|m| SyntaxError { message: m })?;
+    Ok(Path(p))
+}
+
+pub struct Path(ast::Path);
+
+impl Path {
+    pub fn find<'a>(&'a self, document: &'a Value) -> Result<Vec<&'a Value>, FindError> {
+        Ok(self.0.find(document).collect())
+    }
 }

src/lib.rs

@@ -8,7 +8,6 @@ extern crate pest;
 #[macro_use]
 extern crate pest_derive;
 
+pub mod ast;
 pub mod jsonpath;
-mod matchers;
 mod parser;
-pub mod path;