feat: add report formatter trait for custom reporter output (#158)

* Add report formatter concept for customization of report output

* Add default implementation

* Use `format_external` in `format_terms`

* Add previous example

* Use new formatter

* Add docs to trait method

* Clippy

* Remove outdated comment in example

* Use generic

examples/unsat_root_message_no_version.rs

@@ -0,0 +1,146 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use pubgrub::error::PubGrubError;
+use pubgrub::range::Range;
+use pubgrub::report::Reporter;
+use pubgrub::solver::{resolve, OfflineDependencyProvider};
+use pubgrub::version::SemanticVersion;
+
+use pubgrub::report::{DefaultStringReporter, External, ReportFormatter};
+use pubgrub::term::Term;
+use pubgrub::type_aliases::Map;
+use std::fmt::{self, Display};
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum Package {
+    Root,
+    Package(String),
+}
+
+impl Display for Package {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Package::Root => write!(f, "root"),
+            Package::Package(name) => write!(f, "{}", name),
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+struct CustomReportFormatter;
+
+impl ReportFormatter<Package, Range<SemanticVersion>> for CustomReportFormatter {
+    type Output = String;
+
+    fn format_terms(&self, terms: &Map<Package, Term<Range<SemanticVersion>>>) -> String {
+        let terms_vec: Vec<_> = terms.iter().collect();
+        match terms_vec.as_slice() {
+            [] => "version solving failed".into(),
+            [(package @ Package::Root, Term::Positive(_))] => {
+                format!("{package} is forbidden")
+            }
+            [(package @ Package::Root, Term::Negative(_))] => {
+                format!("{package} is mandatory")
+            }
+            [(package @ Package::Package(_), Term::Positive(range))] => {
+                format!("{package} {range} is forbidden")
+            }
+            [(package @ Package::Package(_), Term::Negative(range))] => {
+                format!("{package} {range} is mandatory")
+            }
+            [(p1, Term::Positive(r1)), (p2, Term::Negative(r2))] => {
+                External::FromDependencyOf(p1, r1.clone(), p2, r2.clone()).to_string()
+            }
+            [(p1, Term::Negative(r1)), (p2, Term::Positive(r2))] => {
+                External::FromDependencyOf(p2, r2.clone(), p1, r1.clone()).to_string()
+            }
+            slice => {
+                let str_terms: Vec<_> = slice.iter().map(|(p, t)| format!("{p} {t}")).collect();
+                str_terms.join(", ") + " are incompatible"
+            }
+        }
+    }
+
+    fn format_external(&self, external: &External<Package, Range<SemanticVersion>>) -> String {
+        match external {
+            External::NotRoot(package, version) => {
+                format!("we are solving dependencies of {package} {version}")
+            }
+            External::NoVersions(package, set) => {
+                if set == &Range::full() {
+                    format!("there is no available version for {package}")
+                } else {
+                    format!("there is no version of {package} in {set}")
+                }
+            }
+            External::UnavailableDependencies(package, set) => {
+                if set == &Range::full() {
+                    format!("dependencies of {package} are unavailable")
+                } else {
+                    format!("dependencies of {package} at version {set} are unavailable")
+                }
+            }
+            External::FromDependencyOf(package, package_set, dependency, dependency_set) => {
+                if package_set == &Range::full() && dependency_set == &Range::full() {
+                    format!("{package} depends on {dependency}")
+                } else if package_set == &Range::full() {
+                    format!("{package} depends on {dependency} {dependency_set}")
+                } else if dependency_set == &Range::full() {
+                    if matches!(package, Package::Root) {
+                        // Exclude the dummy version for root packages
+                        format!("{package} depends on {dependency}")
+                    } else {
+                        format!("{package} {package_set} depends on {dependency}")
+                    }
+                } else {
+                    if matches!(package, Package::Root) {
+                        // Exclude the dummy version for root packages
+                        format!("{package} depends on {dependency} {dependency_set}")
+                    } else {
+                        format!("{package} {package_set} depends on {dependency} {dependency_set}")
+                    }
+                }
+            }
+        }
+    }
+}
+
+fn main() {
+    let mut dependency_provider =
+        OfflineDependencyProvider::<Package, Range<SemanticVersion>>::new();
+    // Define the root package with a dependency on a package we do not provide
+    dependency_provider.add_dependencies(
+        Package::Root,
+        (0, 0, 0),
+        vec![(
+            Package::Package("foo".to_string()),
+            Range::singleton((1, 0, 0)),
+        )],
+    );
+
+    // Run the algorithm
+    match resolve(&dependency_provider, Package::Root, (0, 0, 0)) {
+        Ok(sol) => println!("{:?}", sol),
+        Err(PubGrubError::NoSolution(derivation_tree)) => {
+            eprintln!("No solution.\n");
+
+            eprintln!("### Default report:");
+            eprintln!("```");
+            eprintln!("{}", DefaultStringReporter::report(&derivation_tree));
+            eprintln!("```\n");
+
+            eprintln!("### Report with custom formatter:");
+            eprintln!("```");
+            eprintln!(
+                "{}",
+                DefaultStringReporter::report_with_formatter(
+                    &derivation_tree,
+                    &CustomReportFormatter
+                )
+            );
+            eprintln!("```");
+            std::process::exit(1);
+        }
+        Err(err) => panic!("{:?}", err),
+    };
+}

src/internal/incompatibility.rs

@@ -9,7 +9,9 @@ use std::fmt;
 use crate::internal::arena::{Arena, Id};
 use crate::internal::small_map::SmallMap;
 use crate::package::Package;
-use crate::report::{DefaultStringReporter, DerivationTree, Derived, External};
+use crate::report::{
+    DefaultStringReportFormatter, DerivationTree, Derived, External, ReportFormatter,
+};
 use crate::term::{self, Term};
 use crate::version_set::VersionSet;
 
@@ -251,7 +253,7 @@ impl<P: Package, VS: VersionSet> fmt::Display for Incompatibility<P, VS> {
         write!(
             f,
             "{}",
-            DefaultStringReporter::string_terms(&self.package_terms.as_map())
+            DefaultStringReportFormatter.format_terms(&self.package_terms.as_map())
         )
     }
 }