We have dependency resolution failure explanations!

catalog.json

@@ -73,6 +73,13 @@
                     "ref": "0.1.2"
                 },
                 "depends": {}
+            },
+            "0.2.0": {
+                "git": {
+                    "url": "https://github.com/vector-of-bool/pubgrub.git",
+                    "ref": "0.2.0"
+                },
+                "depends": {}
             }
         }
     }

package.dds

@@ -8,6 +8,6 @@ Depends: nlohmann-json 3.7.1
 Depends: neo-sqlite3 0.2.2
 Depends: neo-fun 0.1.0
 Depends: semver 0.2.1
-Depends: pubgrub 0.1.2
+Depends: pubgrub 0.2.0
 
 Test-Driver: Catch-Main

src/dds/deps.cpp

@@ -56,4 +56,49 @@ dependency_manifest dependency_manifest::from_file(path_ref fpath) {
         },
         lm::reject_unknown());
     return ret;
+}
+
+namespace {
+
+std::string iv_string(const pubgrub::interval_set<semver::version>::interval_type& iv) {
+    if (iv.high == semver::version::max_version()) {
+        return ">=" + iv.low.to_string();
+    }
+    if (iv.low == semver::version()) {
+        return "<" + iv.high.to_string();
+    }
+    return iv.low.to_string() + " < " + iv.high.to_string();
+}
+
+} // namespace
+
+std::string dependency::to_string() const noexcept {
+    std::stringstream strm;
+    strm << name << "@";
+    if (versions.num_intervals() == 1) {
+        auto iv = *versions.iter_intervals().begin();
+        if (iv.high == iv.low.next_after()) {
+            strm << iv.low.to_string();
+            return strm.str();
+        }
+        if (iv.low == semver::version() && iv.high == semver::version::max_version()) {
+            return name;
+        }
+        strm << "[" << iv_string(iv) << "]";
+        return strm.str();
+    }
+
+    strm << "[";
+    auto iv_it = versions.iter_intervals();
+    auto it    = iv_it.begin();
+    const auto stop  = iv_it.end();
+    while (it != stop) {
+        strm << "(" << iv_string(*it) << ")";
+        ++it;
+        if (it != stop) {
+            strm << " || ";
+        }
+    }
+    strm << "]";
+    return strm.str();
 }

src/dds/deps.hpp

@@ -17,6 +17,8 @@ struct dependency {
     version_range_set versions;
 
     static dependency parse_depends_string(std::string_view str);
+
+    std::string to_string() const noexcept;
 };
 
 /**

src/dds/solve/solve.cpp

@@ -4,6 +4,9 @@
 
 #include <range/v3/range/conversion.hpp>
 #include <range/v3/view/transform.hpp>
+#include <spdlog/spdlog.h>
+
+#include <sstream>
 
 using namespace dds;
 
@@ -51,6 +54,11 @@ struct req_type {
     friend bool operator==(req_ref lhs, req_ref rhs) noexcept {
         return lhs.dep.name == rhs.dep.name && lhs.dep.versions == rhs.dep.versions;
     }
+
+    friend std::ostream& operator<<(std::ostream& out, req_ref self) noexcept {
+        out << self.dep.to_string();
+        return out;
+    }
 };
 
 auto as_pkg_id(const req_type& req) {
@@ -89,6 +97,51 @@ struct solver_provider {
     }
 };
 
+using solve_fail_exc = pubgrub::solve_failure_type_t<req_type>;
+
+struct explainer {
+    std::stringstream strm;
+    bool              at_head = true;
+
+    void put(pubgrub::explain::no_solution) { strm << "Dependencies cannot be satisfied"; }
+
+    void put(pubgrub::explain::dependency<req_type> dep) {
+        strm << dep.dependent << " requires " << dep.dependency << '.';
+    }
+
+    void put(pubgrub::explain::unavailable<req_type> un) {
+        strm << un.requirement << " is not available";
+    }
+
+    void put(pubgrub::explain::conflict<req_type> cf) {
+        strm << cf.a << " conflicts with " << cf.b;
+    }
+
+    void put(pubgrub::explain::needed<req_type> req) { strm << req.requirement << " is required"; }
+
+    void put(pubgrub::explain::disallowed<req_type> dis) {
+        strm << dis.requirement << " cannot be used";
+    }
+
+    template <typename T>
+    void operator()(pubgrub::explain::premise<T> pr) {
+        strm.str("");
+        put(pr.value);
+        spdlog::error("{} {},", at_head ? "┌─ Given that" : "│         and", strm.str());
+        at_head = false;
+    }
+
+    template <typename T>
+    void operator()(pubgrub::explain::conclusion<T> cncl) {
+        at_head = true;
+        strm.str("");
+        put(cncl.value);
+        spdlog::error("╘═  Then: {}.", strm.str());
+    }
+
+    void operator()(pubgrub::explain::separator) { spdlog::error(""); }
+};
+
 }  // namespace
 
 std::vector<package_id> dds::solve(const std::vector<dependency>& deps,
@@ -97,6 +150,12 @@ std::vector<package_id> dds::solve(const std::vector<dependency>& deps,
     auto wrap_req
         = deps | ranges::v3::views::transform([](const dependency& dep) { return req_type{dep}; });
 
-    auto solution = pubgrub::solve(wrap_req, solver_provider{pkgs_prov, deps_prov});
-    return solution | ranges::views::transform(as_pkg_id) | ranges::to_vector;
+    try {
+        auto solution = pubgrub::solve(wrap_req, solver_provider{pkgs_prov, deps_prov});
+        return solution | ranges::views::transform(as_pkg_id) | ranges::to_vector;
+    } catch (const solve_fail_exc& failure) {
+        spdlog::error("Dependency resolution has failed! Explanation:");
+        pubgrub::generate_explaination(failure, explainer());
+        throw;
+    }
 }