* New query option: `--compare-versions' or `-c' to compare installed

  versions to available versions, or vice versa.

  For example, the following compares installed versions to available
  versions:

    $ nix-env -qc
    autoconf-2.59            = 2.59
    automake-1.9.4           < 1.9.6
    f-spot-0.0.10            - ?
    firefox-1.0.4            < 1.0.7
    ...

  I.e., there are newer versions available (in the current default Nix
  expression) for Automake and Firefox, but not for Autoconf, and
  F-Spot is missing altogether.

  Conversely, the available versions can be compared to the installed
  versions:

    $ nix-env -qac
    autoconf-2.59                  = 2.59
    automake-1.9.6                 > 1.9.4
    bash-3.0                       - ?
    firefox-1.0.7                  > 1.0.4
    ...

  Note that bash is available but no version of it is installed.

  If multiple versions are available for comparison, then the highest
  is used.  E.g., if Subversion 1.2.0 is installed, and Subversion
  1.1.4 and 1.2.3 are available, then `nix-env -qc' will print `<
  1.2.3', not `> 1.1.4'.

  If higher versions are available, the version column is printed in
  red (using ANSI escape codes).

2 files changed, 89 insertions, 19 deletions

diff --git a/src/nix-env/help.txt b/src/nix-env/help.txt
index 4fc1602840..6ca5b3a3b8 100644
--- a/src/nix-env/help.txt
+++ b/src/nix-env/help.txt
@@ -36,10 +36,12 @@ Upgrade flags:
 
 Query types:
 
-  --name: print derivation names (default)
+  --status / -s: print installed/present status
+  --no-name: hide derivation names
+  --system: print the platform type of the derivation 
+  --compare-versions / -c: compare version to available or installed
   --drv-path: print path of derivation
   --out-path: print path of derivation output
-  --status / -s: print installed/present status
 
 Query sources:
 
diff --git a/src/nix-env/main.cc b/src/nix-env/main.cc
index 96bbce5cf0..6d542c9b6d 100644
--- a/src/nix-env/main.cc
+++ b/src/nix-env/main.cc
@@ -668,6 +668,53 @@ void printTable(Table & table)
 }
 
 
+/* This function compares the version of a element against the
+   versions in the given set of elements.  `cvLess' means that only
+   lower versions are in the set, `cvEqual' means that at most an
+   equal version is in the set, and `cvGreater' means that there is at
+   least one element with a higher version in the set.  `cvUnavail'
+   means that there are no elements with the same name in the set. */
+
+typedef enum { cvLess, cvEqual, cvGreater, cvUnavail } VersionDiff;
+
+static VersionDiff compareVersionAgainstSet(
+    const UserEnvElem & elem, const UserEnvElems & elems, string & version)
+{
+    DrvName name(elem.name);
+    
+    VersionDiff diff = cvUnavail;
+    version = "?";
+    
+    for (UserEnvElems::const_iterator i = elems.begin(); i != elems.end(); ++i) {
+        DrvName name2(i->second.name);
+        if (name.name == name2.name) {
+            int d = compareVersions(name.version, name2.version);
+            if (d < 0) {
+                diff = cvGreater;
+                version = name2.version;
+            }
+            else if (diff != cvGreater && d == 0) {
+                diff = cvEqual;
+                version = name2.version;
+            }
+            else if (diff != cvGreater && diff != cvEqual && d > 0) {
+                diff = cvLess;
+                if (version == "" || compareVersions(version, name2.version) < 0)
+                    version = name2.version;
+            }
+        }
+    }
+
+    return diff;
+}
+
+
+static string colorString(const string & s)
+{
+    return "\e[1;31m" + s + "\e[0m";
+}
+
+
 static void opQuery(Globals & globals,
     Strings opFlags, Strings opArgs)
 {
@@ -676,6 +723,7 @@ static void opQuery(Globals & globals,
     bool printSystem = false;
     bool printDrvPath = false;
     bool printOutPath = false;
+    bool compareVersions = false;
 
     enum { sInstalled, sAvailable } source = sInstalled;
 
@@ -686,50 +734,50 @@ static void opQuery(Globals & globals,
         if (*i == "--status" || *i == "-s") printStatus = true;
         else if (*i == "--no-name") printName = false;
         else if (*i == "--system") printSystem = true;
+        else if (*i == "--compare-versions" || *i == "-c") compareVersions = true;
         else if (*i == "--drv-path") printDrvPath = true;
         else if (*i == "--out-path") printOutPath = true;
         else if (*i == "--installed") source = sInstalled;
         else if (*i == "--available" || *i == "-a") source = sAvailable;
         else throw UsageError(format("unknown flag `%1%'") % *i);
 
-    /* Obtain derivation information from the specified source. */
-    UserEnvElems elems;
-
-    switch (source) {
+    if (opArgs.size() != 0) throw UsageError("no arguments expected");
 
-        case sInstalled:
-            elems = queryInstalled(globals.state, globals.profile);
-            break;
+    
+    /* Obtain derivation information from the specified source. */
+    UserEnvElems availElems, installedElems;
 
-        case sAvailable: {
-            loadDerivations(globals.state, globals.instSource.nixExprPath,
-                globals.instSource.systemFilter, elems);
-            break;
-        }
+    if (source == sInstalled || compareVersions) {
+        installedElems = queryInstalled(globals.state, globals.profile);
+    }
 
-        default: abort();
+    if (source == sAvailable || compareVersions) {
+        loadDerivations(globals.state, globals.instSource.nixExprPath,
+            globals.instSource.systemFilter, availElems);
     }
 
-    if (opArgs.size() != 0) throw UsageError("no arguments expected");
+    UserEnvElems & elems(source == sInstalled ? installedElems : availElems);
+    UserEnvElems & otherElems(source == sInstalled ? availElems : installedElems);
 
+    
     /* Sort them by name. */
     vector<UserEnvElem> elems2;
     for (UserEnvElems::iterator i = elems.begin(); i != elems.end(); ++i)
         elems2.push_back(i->second);
     sort(elems2.begin(), elems2.end(), cmpElemByName);
 
+    
     /* We only need to know the installed paths when we are querying
        the status of the derivation. */
     PathSet installed; /* installed paths */
     
     if (printStatus) {
-        UserEnvElems installedElems; 
-        installedElems = queryInstalled(globals.state, globals.profile);
         for (UserEnvElems::iterator i = installedElems.begin();
              i != installedElems.end(); ++i)
             installed.insert(i->second.queryOutPath(globals.state));
     }
-            
+
+    
     /* Print the desired columns. */
     Table table;
     
@@ -751,6 +799,26 @@ static void opQuery(Globals & globals,
 
         if (printSystem) columns.push_back(i->system);
 
+        if (compareVersions) {
+            /* Compare this element against the versions of the same
+               named packages in either the set of available elements,
+               or the set of installed elements.  !!! This is O(N *
+               M), should be O(N * lg M). */
+            string version;
+            VersionDiff diff = compareVersionAgainstSet(*i, otherElems, version);
+            char ch;
+            switch (diff) {
+                case cvLess: ch = '>'; break;
+                case cvEqual: ch = '='; break;
+                case cvGreater: ch = '<'; break;
+                case cvUnavail: ch = '-'; break;
+                default: abort();
+            }
+            string column = (string) "" + ch + " " + version;
+            if (diff == cvGreater) column = colorString(column);
+            columns.push_back(column);
+        }
+
         if (printDrvPath) columns.push_back(
             i->queryDrvPath(globals.state) == ""
             ? "-" : i->queryDrvPath(globals.state));