1 files changed, 135 insertions, 3 deletions
diff --git a/tests/test_utils/swc_utils.rs b/tests/test_utils/swc_utils.rs
index 1e1a526..5a45af6 100644
--- a/tests/test_utils/swc_utils.rs
+++ b/tests/test_utils/swc_utils.rs
@@ -1,8 +1,140 @@
-extern crate swc_common;
-extern crate swc_ecma_ast;
+//! Lots of helpers for dealing with SWC, particularly from unist.
 
-use swc_common::DUMMY_SP;
+use micromark::{
+    mdast::Stop,
+    unist::{Point, Position},
+    Location,
+};
+
+use swc_common::{BytePos, Span, SyntaxContext, DUMMY_SP};
 use swc_ecma_ast::{BinExpr, BinaryOp, Expr, Ident, MemberExpr, MemberProp};
+use swc_ecma_visit::{noop_visit_mut_type, VisitMut};
+
+/// Turn a unist position, into an SWC span, of two byte positions.
+///
+/// > 👉 **Note**: SWC byte positions are offset by one: they are `0` when they
+/// > are missing or incremented by `1` when valid.
+pub fn position_to_span(position: Option<&Position>) -> Span {
+    position.map_or(DUMMY_SP, |d| Span {
+        lo: point_to_bytepos(&d.start),
+        hi: point_to_bytepos(&d.end),
+        ctxt: SyntaxContext::empty(),
+    })
+}
+
+/// Turn an SWC span, of two byte positions, into a unist position.
+///
+/// This assumes the span comes from a fixed tree, or is a dummy.
+///
+/// > 👉 **Note**: SWC byte positions are offset by one: they are `0` when they
+/// > are missing or incremented by `1` when valid.
+pub fn span_to_position(span: &Span, location: Option<&Location>) -> Option<Position> {
+    let lo = span.lo.0 as usize;
+    let hi = span.hi.0 as usize;
+
+    if lo > 0 && hi > 0 {
+        if let Some(location) = location {
+            if let Some(start) = location.to_point(lo - 1) {
+                if let Some(end) = location.to_point(hi - 1) {
+                    return Some(Position { start, end });
+                }
+            }
+        }
+    }
+
+    None
+}
+
+/// Turn a unist point into an SWC byte position.
+///
+/// > 👉 **Note**: SWC byte positions are offset by one: they are `0` when they
+/// > are missing or incremented by `1` when valid.
+pub fn point_to_bytepos(point: &Point) -> BytePos {
+    BytePos(point.offset as u32 + 1)
+}
+
+/// Turn an SWC byte position into a unist point.
+///
+/// This assumes the byte position comes from a fixed tree, or is a dummy.
+///
+/// > 👉 **Note**: SWC byte positions are offset by one: they are `0` when they
+/// > are missing or incremented by `1` when valid.
+pub fn bytepos_to_point(bytepos: &BytePos, location: Option<&Location>) -> Option<Point> {
+    let pos = bytepos.0 as usize;
+
+    if pos > 0 {
+        if let Some(location) = location {
+            return location.to_point(pos - 1);
+        }
+    }
+
+    None
+}
+
+/// Prefix an error message with an optional point.
+pub fn prefix_error_with_point(reason: String, point: Option<&Point>) -> String {
+    if let Some(point) = point {
+        format!("{}: {}", point_to_string(point), reason)
+    } else {
+        reason
+    }
+}
+
+/// Serialize a unist position for humans.
+pub fn position_to_string(position: &Position) -> String {
+    format!(
+        "{}-{}",
+        point_to_string(&position.start),
+        point_to_string(&position.end)
+    )
+}
+
+/// Serialize a unist point for humans.
+pub fn point_to_string(point: &Point) -> String {
+    format!("{}:{}", point.line, point.column)
+}
+
+/// Visitor to fix SWC byte positions.
+///
+/// This assumes the byte position comes from an **unfixed** tree.
+///
+/// > 👉 **Note**: SWC byte positions are offset by one: they are `0` when they
+/// > are missing or incremented by `1` when valid.
+#[derive(Debug, Default, Clone)]
+pub struct RewriteContext<'a> {
+    pub prefix_len: usize,
+    pub stops: &'a [Stop],
+    pub location: Option<&'a Location>,
+}
+
+impl<'a> VisitMut for RewriteContext<'a> {
+    noop_visit_mut_type!();
+
+    // Rewrite spans.
+    fn visit_mut_span(&mut self, span: &mut Span) {
+        let mut result = DUMMY_SP;
+        let lo_rel = span.lo.0 as usize;
+        let hi_rel = span.hi.0 as usize;
+
+        if lo_rel > self.prefix_len && hi_rel > self.prefix_len {
+            if let Some(lo_abs) =
+                Location::relative_to_absolute(self.stops, lo_rel - 1 - self.prefix_len)
+            {
+                if let Some(hi_abs) =
+                    Location::relative_to_absolute(self.stops, hi_rel - 1 - self.prefix_len)
+                {
+                    result = Span {
+                        lo: BytePos(lo_abs as u32 + 1),
+                        hi: BytePos(hi_abs as u32 + 1),
+                        ctxt: SyntaxContext::empty(),
+                    };
+                }
+            }
+        }
+
+        *span = result;
+    }
+}
 
 /// Generate an ident.
 ///