diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index d0e6635fe0..49b7699a4e 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -68,6 +68,15 @@ core.fsmonitor::
 	avoiding unnecessary processing of files that have not changed.
 	See the "fsmonitor-watchman" section of linkgit:githooks[5].
 
+core.virtualWorkDir::
+	Please regard this as an experimental feature.
+	If set to true, utilize the virtual-work-dir hook to identify all
+	files and directories that are present in the working directory.
+	Git will only track and update files listed in the virtual work
+	directory.  Using the virtual work directory will supersede the
+	sparse-checkout settings which will be ignored.
+	See the "virtual-work-dir" section of linkgit:githooks[6].
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 959044347e..9888d504b4 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -485,6 +485,29 @@ The exit status determines whether git will use the data from the
 hook to limit its search.  On error, it will fall back to verifying
 all files and folders.
 
+virtual-work-dir
+~~~~~~~~~~~~~~~~
+
+Please regard this as an experimental feature.
+
+The "Virtual Work Directory" hook allows populating the working directory
+sparsely. The virtual work directory data is typically automatically
+generated by an external process.  Git will limit what files it checks for
+changes as well as which directories are checked for untracked files based
+on the path names given. Git will also only update those files listed in the
+virtual work directory.
+
+The hook is invoked when the configuration option core.virtualWorkDir is
+set to true.  The hook takes one argument, a version (currently 1).
+
+The hook should output to stdout the list of all files in the working
+directory that git should track.  The paths are relative to the root
+of the working directory and are separated by a single NUL.  Full paths
+('dir1/a.txt') as well as directories are supported (ie 'dir1/').
+
+The exit status determines whether git will use the data from the
+hook.  On error, git will abort the command with an error message.
+
 p4-pre-submit
 ~~~~~~~~~~~~~
 
diff --git a/Makefile b/Makefile
index 1a44c811aa..061f1ab954 100644
--- a/Makefile
+++ b/Makefile
@@ -1012,6 +1012,7 @@ LIB_OBJS += utf8.o
 LIB_OBJS += varint.o
 LIB_OBJS += version.o
 LIB_OBJS += versioncmp.o
+LIB_OBJS += virtualworkdir.o
 LIB_OBJS += walker.o
 LIB_OBJS += wildmatch.o
 LIB_OBJS += worktree.o
diff --git a/cache.h b/cache.h
index ca36b44ee0..39650e6efd 100644
--- a/cache.h
+++ b/cache.h
@@ -886,6 +886,7 @@ extern char *git_replace_ref_base;
 extern int fsync_object_files;
 extern int core_preload_index;
 extern int core_apply_sparse_checkout;
+extern int core_virtualworkdir;
 extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
diff --git a/config.c b/config.c
index ff521eb27a..fc0d51aa69 100644
--- a/config.c
+++ b/config.c
@@ -1325,7 +1325,11 @@ static int git_default_core_config(const char *var, const char *value, void *cb)
 	}
 
 	if (!strcmp(var, "core.sparsecheckout")) {
-		core_apply_sparse_checkout = git_config_bool(var, value);
+		/* virtual working directory relies on the sparse checkout logic so force it on */
+		if (core_virtualworkdir)
+			core_apply_sparse_checkout = 1;
+		else
+			core_apply_sparse_checkout = git_config_bool(var, value);
 		return 0;
 	}
 
@@ -2315,6 +2319,32 @@ int git_config_get_index_threads(int *dest)
 	return 1;
 }
 
+int git_config_get_virtualworkdir(void)
+{
+	git_config_get_bool("core.virtualworkdir", &core_virtualworkdir);
+	if (core_virtualworkdir) {
+		/*
+		 * Some git commands spawn helpers and redirect the index to a different
+		 * location.  These include "difftool -d" and the sequencer
+		 * (i.e. `git rebase -i`, `git cherry-pick` and `git revert`) and others.
+		 * In those instances we don't want to update their temporary index with
+		 * our virtualization data.
+		 */
+		char *default_index_file = xstrfmt("%s/%s", the_repository->gitdir, "index");
+		int should_run_hook = !strcmp(default_index_file, the_repository->index_file);
+
+		free(default_index_file);
+		if (should_run_hook) {
+			/* virtual working directory relies on the sparse checkout logic so force it on */
+			core_apply_sparse_checkout = 1;
+			return core_virtualworkdir;
+		}
+		core_virtualworkdir = 0;
+	}
+
+	return core_virtualworkdir;
+}
+
 NORETURN
 void git_die_config_linenr(const char *key, const char *filename, int linenr)
 {
diff --git a/config.h b/config.h
index ee5d3fa7b4..e89590603c 100644
--- a/config.h
+++ b/config.h
@@ -251,6 +251,7 @@ extern int git_config_get_untracked_cache(void);
 extern int git_config_get_split_index(void);
 extern int git_config_get_max_percent_split_change(void);
 extern int git_config_get_fsmonitor(void);
+extern int git_config_get_virtualworkdir(void);
 
 /* This dies if the configured or default date is in the future */
 extern int git_config_get_expiry(const char *key, const char **output);
diff --git a/dir.c b/dir.c
index ab6477d777..987a3eb17f 100644
--- a/dir.c
+++ b/dir.c
@@ -21,6 +21,7 @@
 #include "ewah/ewok.h"
 #include "fsmonitor.h"
 #include "submodule-config.h"
+#include "virtualworkdir.h"
 
 /*
  * Tells read_directory_recursive how a file or directory should be treated.
@@ -1116,6 +1117,14 @@ int is_excluded_from_list(const char *pathname,
 			  struct exclude_list *el, struct index_state *istate)
 {
 	struct exclude *exclude;
+
+	if (core_virtualworkdir) {
+		if (*dtype == DT_UNKNOWN)
+			*dtype = get_dtype(NULL, istate, pathname, pathlen);
+		if (is_excluded_from_virtualworkdir(pathname, pathlen, *dtype) > 0)
+			return 1;
+	}
+
 	exclude = last_exclude_matching_from_list(pathname, pathlen, basename,
 						  dtype, el, istate);
 	if (exclude)
@@ -1331,8 +1340,16 @@ struct exclude *last_exclude_matching(struct dir_struct *dir,
 int is_excluded(struct dir_struct *dir, struct index_state *istate,
 		const char *pathname, int *dtype_p)
 {
-	struct exclude *exclude =
-		last_exclude_matching(dir, istate, pathname, dtype_p);
+	struct exclude *exclude;
+
+	if (core_virtualworkdir) {
+		if (*dtype_p == DT_UNKNOWN)
+			*dtype_p = get_dtype(NULL, istate, pathname, strlen(pathname));
+		if (is_excluded_from_virtualworkdir(pathname, strlen(pathname), *dtype_p) > 0)
+			return 1;
+	}
+
+	exclude = last_exclude_matching(dir, istate, pathname, dtype_p);
 	if (exclude)
 		return exclude->flags & EXC_FLAG_NEGATIVE ? 0 : 1;
 	return 0;
@@ -1685,6 +1702,9 @@ static enum path_treatment treat_one_path(struct dir_struct *dir,
 	if (dtype != DT_DIR && has_path_in_index)
 		return path_none;
 
+	if (is_excluded_from_virtualworkdir(path->buf, path->len, dtype) > 0)
+		return path_excluded;
+
 	/*
 	 * When we are looking at a directory P in the working tree,
 	 * there are three cases:
@@ -2025,6 +2045,8 @@ static enum path_treatment read_directory_recursive(struct dir_struct *dir,
 		/* add the path to the appropriate result list */
 		switch (state) {
 		case path_excluded:
+			if (is_excluded_from_virtualworkdir(path.buf, path.len, DT_DIR) > 0)
+				break;
 			if (dir->flags & DIR_SHOW_IGNORED)
 				dir_add_name(dir, istate, path.buf, path.len);
 			else if ((dir->flags & DIR_SHOW_IGNORED_TOO) ||
diff --git a/environment.c b/environment.c
index 3465597707..bc0cef4506 100644
--- a/environment.c
+++ b/environment.c
@@ -69,6 +69,7 @@ enum object_creation_mode object_creation_mode = OBJECT_CREATION_MODE;
 char *notes_ref_name;
 int grafts_replace_parents = 1;
 int core_apply_sparse_checkout;
+int core_virtualworkdir;
 int merge_log_config = -1;
 int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
 unsigned long pack_size_limit_cfg;
diff --git a/read-cache.c b/read-cache.c
index bd45dc3e24..a2c8027977 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -25,6 +25,7 @@
 #include "fsmonitor.h"
 #include "thread-utils.h"
 #include "progress.h"
+#include "virtualworkdir.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -1894,6 +1895,7 @@ static void post_read_index_from(struct index_state *istate)
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
 	tweak_fsmonitor(istate);
+	apply_virtualworkdir(istate);
 }
 
 static size_t estimate_cache_size_from_compressed(unsigned int entries)
diff --git a/t/t1092-virtualworkdir.sh b/t/t1092-virtualworkdir.sh
new file mode 100755
index 0000000000..0cdfe9b362
--- /dev/null
+++ b/t/t1092-virtualworkdir.sh
@@ -0,0 +1,393 @@
+#!/bin/sh
+
+test_description='virtual work directory tests'
+
+. ./test-lib.sh
+
+# We need total control of the virtual work directory hook
+sane_unset GIT_TEST_VIRTUALWORKDIR
+
+clean_repo () {
+	rm .git/index &&
+	git -c core.virtualworkdir=false reset --hard HEAD &&
+	git -c core.virtualworkdir=false clean -fd &&
+	touch untracked.txt &&
+	touch dir1/untracked.txt &&
+	touch dir2/untracked.txt
+}
+
+test_expect_success 'setup' '
+	mkdir -p .git/hooks/ &&
+	cat > .gitignore <<-\EOF &&
+		.gitignore
+		expect*
+		actual*
+	EOF
+	touch file1.txt &&
+	touch file2.txt &&
+	mkdir -p dir1 &&
+	touch dir1/file1.txt &&
+	touch dir1/file2.txt &&
+	mkdir -p dir2 &&
+	touch dir2/file1.txt &&
+	touch dir2/file2.txt &&
+	git add . &&
+	git commit -m "initial" &&
+	git config --local core.virtualworkdir true
+'
+
+test_expect_success 'test hook parameters and version' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		if test "$#" -ne 1
+		then
+			echo "$0: Exactly 1 argument expected" >&2
+			exit 2
+		fi
+
+		if test "$1" != 1
+		then
+			echo "$0: Unsupported hook version." >&2
+			exit 1
+		fi
+	EOF
+	git status &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		exit 3
+	EOF
+	test_must_fail git status
+'
+
+test_expect_success 'verify status is clean' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir2/file1.txt\0"
+	EOF
+	rm -f .git/index &&
+	git checkout -f &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir2/file1.txt\0"
+		printf "dir1/file1.txt\0"
+		printf "dir1/file2.txt\0"
+	EOF
+	git status > actual &&
+	cat > expected <<-\EOF &&
+		On branch master
+		nothing to commit, working tree clean
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify skip-worktree bit is set for absolute path' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file1.txt\0"
+	EOF
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		H dir1/file1.txt
+		S dir1/file2.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify skip-worktree bit is cleared for absolute path' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file2.txt\0"
+	EOF
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		S dir1/file1.txt
+		H dir1/file2.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify folder wild cards' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/\0"
+	EOF
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		H dir1/file1.txt
+		H dir1/file2.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify folders not included are ignored' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file1.txt\0"
+		printf "dir1/file2.txt\0"
+	EOF
+	mkdir -p dir1/dir2 &&
+	touch dir1/a &&
+	touch dir1/b &&
+	touch dir1/dir2/a &&
+	touch dir1/dir2/b &&
+	git add . &&
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		H dir1/file1.txt
+		H dir1/file2.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify including one file doesnt include the rest' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file1.txt\0"
+		printf "dir1/file2.txt\0"
+		printf "dir1/dir2/a\0"
+	EOF
+	mkdir -p dir1/dir2 &&
+	touch dir1/a &&
+	touch dir1/b &&
+	touch dir1/dir2/a &&
+	touch dir1/dir2/b &&
+	git add . &&
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		H dir1/dir2/a
+		H dir1/file1.txt
+		H dir1/file2.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify files not listed are ignored by git clean -f -x' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "untracked.txt\0"
+		printf "dir1/\0"
+	EOF
+	mkdir -p dir3 &&
+	touch dir3/untracked.txt &&
+	git clean -f -x &&
+	test -f file1.txt &&
+	test -f file2.txt &&
+	test ! -f untracked.txt &&
+	test -d dir1 &&
+	test -f dir1/file1.txt &&
+	test -f dir1/file2.txt &&
+	test ! -f dir1/untracked.txt &&
+	test -f dir2/file1.txt &&
+	test -f dir2/file2.txt &&
+	test -f dir2/untracked.txt &&
+	test -d dir3 &&
+	test -f dir3/untracked.txt
+'
+
+test_expect_success 'verify files not listed are ignored by git clean -f -d -x' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "untracked.txt\0"
+		printf "dir1/\0"
+		printf "dir3/\0"
+	EOF
+	mkdir -p dir3 &&
+	touch dir3/untracked.txt &&
+	git clean -f -d -x &&
+	test -f file1.txt &&
+	test -f file2.txt &&
+	test ! -f untracked.txt &&
+	test -d dir1 &&
+	test -f dir1/file1.txt &&
+	test -f dir1/file2.txt &&
+	test ! -f dir1/untracked.txt &&
+	test -f dir2/file1.txt &&
+	test -f dir2/file2.txt &&
+	test -f dir2/untracked.txt &&
+	test ! -d dir3 &&
+	test ! -f dir3/untracked.txt
+'
+
+test_expect_success 'verify folder entries include all files' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/\0"
+	EOF
+	mkdir -p dir1/dir2 &&
+	touch dir1/a &&
+	touch dir1/b &&
+	touch dir1/dir2/a &&
+	touch dir1/dir2/b &&
+	git status -su > actual &&
+	cat > expected <<-\EOF &&
+		?? dir1/a
+		?? dir1/b
+		?? dir1/dir2/a
+		?? dir1/dir2/b
+		?? dir1/untracked.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'verify case insensitivity of virtual work directory entries' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/a\0"
+		printf "Dir1/Dir2/a\0"
+		printf "DIR2/\0"
+	EOF
+	mkdir -p dir1/dir2 &&
+	touch dir1/a &&
+	touch dir1/b &&
+	touch dir1/dir2/a &&
+	touch dir1/dir2/b &&
+	git -c core.ignorecase=false status -su > actual &&
+	cat > expected <<-\EOF &&
+		?? dir1/a
+	EOF
+	test_cmp expected actual &&
+	git -c core.ignorecase=true status -su > actual &&
+	cat > expected <<-\EOF &&
+		?? dir1/a
+		?? dir1/dir2/a
+		?? dir2/untracked.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'on file created' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file3.txt\0"
+	EOF
+	touch dir1/file3.txt &&
+	git add . &&
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		S dir1/file1.txt
+		S dir1/file2.txt
+		H dir1/file3.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'on file renamed' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file1.txt\0"
+		printf "dir1/file3.txt\0"
+	EOF
+	mv dir1/file1.txt dir1/file3.txt &&
+	git status -su > actual &&
+	cat > expected <<-\EOF &&
+		 D dir1/file1.txt
+		?? dir1/file3.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'on file deleted' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file1.txt\0"
+	EOF
+	rm dir1/file1.txt &&
+	git status -su > actual &&
+	cat > expected <<-\EOF &&
+		 D dir1/file1.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'on file overwritten' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/file1.txt\0"
+	EOF
+	echo "overwritten" > dir1/file1.txt &&
+	git status -su > actual &&
+	cat > expected <<-\EOF &&
+		 M dir1/file1.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'on folder created' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/dir1/\0"
+	EOF
+	mkdir -p dir1/dir1 &&
+	git status -su > actual &&
+	cat > expected <<-\EOF &&
+	EOF
+	test_cmp expected actual &&
+	git clean -fd &&
+	test ! -d "/dir1/dir1"
+'
+
+test_expect_success 'on folder renamed' '
+	clean_repo &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir3/\0"
+		printf "dir1/file1.txt\0"
+		printf "dir1/file2.txt\0"
+		printf "dir3/file1.txt\0"
+		printf "dir3/file2.txt\0"
+	EOF
+	mv dir1 dir3 &&
+	git status -su > actual &&
+	cat > expected <<-\EOF &&
+		 D dir1/file1.txt
+		 D dir1/file2.txt
+		?? dir3/file1.txt
+		?? dir3/file2.txt
+		?? dir3/untracked.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'folder with same prefix as file' '
+	clean_repo &&
+	touch dir1.sln &&
+	write_script .git/hooks/virtual-work-dir <<-\EOF &&
+		printf "dir1/\0"
+		printf "dir1.sln\0"
+	EOF
+	git add dir1.sln &&
+	git ls-files -v > actual &&
+	cat > expected <<-\EOF &&
+		H dir1.sln
+		H dir1/file1.txt
+		H dir1/file2.txt
+		S dir2/file1.txt
+		S dir2/file2.txt
+		S file1.txt
+		S file2.txt
+	EOF
+	test_cmp expected actual
+'
+
+test_done
diff --git a/unpack-trees.c b/unpack-trees.c
index 7570df481b..c6c20c9b61 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -18,6 +18,7 @@
 #include "fsmonitor.h"
 #include "object-store.h"
 #include "fetch-object.h"
+#include "virtualworkdir.h"
 
 /*
  * Error messages expected by scripts out of plumbing commands such as
@@ -1363,6 +1364,14 @@ static int clear_ce_flags_1(struct index_state *istate,
 			continue;
 		}
 
+		/* if it's not in the virtual working directory, exit early */
+		if (core_virtualworkdir) {
+			if (is_included_in_virtualworkdir(ce->name, ce->ce_namelen) > 0)
+				ce->ce_flags &= ~clear_mask;
+			cache++;
+			continue;
+		}
+
 		if (prefix->len && strncmp(ce->name, prefix->buf, prefix->len))
 			break;
 
@@ -1481,12 +1490,16 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	if (!core_apply_sparse_checkout || !o->update)
 		o->skip_sparse_checkout = 1;
 	if (!o->skip_sparse_checkout) {
-		char *sparse = git_pathdup("info/sparse-checkout");
-		if (add_excludes_from_file_to_list(sparse, "", 0, &el, NULL) < 0)
-			o->skip_sparse_checkout = 1;
-		else
+		if (core_virtualworkdir) {
 			o->el = &el;
-		free(sparse);
+		} else {
+			char *sparse = git_pathdup("info/sparse-checkout");
+			if (add_excludes_from_file_to_list(sparse, "", 0, &el, NULL) < 0)
+				o->skip_sparse_checkout = 1;
+			else
+				o->el = &el;
+			free(sparse);
+		}
 	}
 
 	memset(&o->result, 0, sizeof(o->result));
diff --git a/virtualworkdir.c b/virtualworkdir.c
new file mode 100644
index 0000000000..f2c8025bf5
--- /dev/null
+++ b/virtualworkdir.c
@@ -0,0 +1,314 @@
+#include "cache.h"
+#include "config.h"
+#include "dir.h"
+#include "hashmap.h"
+#include "run-command.h"
+#include "virtualworkdir.h"
+
+#define HOOK_INTERFACE_VERSION	(1)
+
+static struct strbuf virtual_workdir_data = STRBUF_INIT;
+static struct hashmap virtual_workdir_hashmap;
+static struct hashmap parent_directory_hashmap;
+
+struct virtualworkdir {
+	struct hashmap_entry ent; /* must be the first member! */
+	const char *pattern;
+	int patternlen;
+};
+
+static unsigned int(*vwdhash)(const void *buf, size_t len);
+static int(*vwdcmp)(const char *a, const char *b, size_t len);
+
+static int vwd_hashmap_cmp(const void *unused_cmp_data,
+	const void *a, const void *b, const void *key)
+{
+	const struct virtualworkdir *vwd1 = a;
+	const struct virtualworkdir *vwd2 = b;
+
+	return vwdcmp(vwd1->pattern, vwd2->pattern, vwd1->patternlen);
+}
+
+static void get_virtual_workdir_data(struct strbuf *vwd_data)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	const char *p;
+	int err;
+
+	strbuf_init(vwd_data, 0);
+
+	p = find_hook("virtual-work-dir");
+	if (!p)
+		die("unable to find virtual-work-dir hook");
+
+	argv_array_push(&cp.args, p);
+	argv_array_pushf(&cp.args, "%d", HOOK_INTERFACE_VERSION);
+	cp.use_shell = 1;
+	cp.dir = get_git_work_tree();
+
+	err = capture_command(&cp, vwd_data, 1024);
+	if (err)
+		die("unable to load virtual working directory");
+}
+
+static int check_includes_hashmap(struct hashmap *map, const char *pattern, int patternlen)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct virtualworkdir vwd;
+	char *slash;
+
+	/* Check straight mapping */
+	strbuf_reset(&sb);
+	strbuf_add(&sb, pattern, patternlen);
+	vwd.pattern = sb.buf;
+	vwd.patternlen = sb.len;
+	hashmap_entry_init(&vwd, vwdhash(vwd.pattern, vwd.patternlen));
+	if (hashmap_get(map, &vwd, NULL)) {
+		strbuf_release(&sb);
+		return 1;
+	}
+
+	/*
+	 * Check to see if it matches a directory or any path
+	 * underneath it.  In other words, 'a/b/foo.txt' will match
+	 * '/', 'a/', and 'a/b/'.
+	 */
+	slash = strchr(sb.buf, '/');
+	while (slash) {
+		vwd.pattern = sb.buf;
+		vwd.patternlen = slash - sb.buf + 1;
+		hashmap_entry_init(&vwd, vwdhash(vwd.pattern, vwd.patternlen));
+		if (hashmap_get(map, &vwd, NULL)) {
+			strbuf_release(&sb);
+			return 1;
+		}
+		slash = strchr(slash + 1, '/');
+	}
+
+	strbuf_release(&sb);
+	return 0;
+}
+
+static void includes_hashmap_add(struct hashmap *map, const char *pattern, const int patternlen)
+{
+	struct virtualworkdir *vwd;
+
+	vwd = xmalloc(sizeof(struct virtualworkdir));
+	vwd->pattern = pattern;
+	vwd->patternlen = patternlen;
+	hashmap_entry_init(vwd, vwdhash(vwd->pattern, vwd->patternlen));
+	hashmap_add(map, vwd);
+}
+
+static void initialize_includes_hashmap(struct hashmap *map, struct strbuf *vwd_data)
+{
+	char *buf, *entry;
+	size_t len;
+	int i;
+
+	/*
+	 * Build a hashmap of the virtual working directory data we can use to look
+	 * for cache entry matches quickly
+	 */
+	vwdhash = ignore_case ? memihash : memhash;
+	vwdcmp = ignore_case ? strncasecmp : strncmp;
+	hashmap_init(map, vwd_hashmap_cmp, NULL, 0);
+
+	entry = buf = vwd_data->buf;
+	len = vwd_data->len;
+	for (i = 0; i < len; i++) {
+		if (buf[i] == '\0') {
+			includes_hashmap_add(map, entry, buf + i - entry);
+			entry = buf + i + 1;
+		}
+	}
+}
+
+/*
+ * Return 1 if the requested item is found in the virtual working directory,
+ * 0 for not found and -1 for undecided.
+ */
+int is_included_in_virtualworkdir(const char *pathname, int pathlen)
+{
+	if (!core_virtualworkdir)
+		return -1;
+
+	if (!virtual_workdir_hashmap.tablesize && virtual_workdir_data.len)
+		initialize_includes_hashmap(&virtual_workdir_hashmap, &virtual_workdir_data);
+	if (!virtual_workdir_hashmap.tablesize)
+		return -1;
+
+	return check_includes_hashmap(&virtual_workdir_hashmap, pathname, pathlen);
+}
+
+static void parent_directory_hashmap_add(struct hashmap *map, const char *pattern, const int patternlen)
+{
+	char *slash;
+	struct virtualworkdir *vwd;
+
+	/*
+	 * Add any directories leading up to the file as the excludes logic
+	 * needs to match directories leading up to the files as well. Detect
+	 * and prevent unnecessary duplicate entries which will be common.
+	 */
+	if (patternlen > 1) {
+		slash = strchr(pattern + 1, '/');
+		while (slash) {
+			vwd = xmalloc(sizeof(struct virtualworkdir));
+			vwd->pattern = pattern;
+			vwd->patternlen = slash - pattern + 1;
+			hashmap_entry_init(vwd, vwdhash(vwd->pattern, vwd->patternlen));
+			if (hashmap_get(map, vwd, NULL))
+				free(vwd);
+			else
+				hashmap_add(map, vwd);
+			slash = strchr(slash + 1, '/');
+		}
+	}
+}
+
+static void initialize_parent_directory_hashmap(struct hashmap *map, struct strbuf *vwd_data)
+{
+	char *buf, *entry;
+	size_t len;
+	int i;
+
+	/*
+	 * Build a hashmap of the parent directories contained in the virtual
+	 * file system data we can use to look for matches quickly
+	 */
+	vwdhash = ignore_case ? memihash : memhash;
+	vwdcmp = ignore_case ? strncasecmp : strncmp;
+	hashmap_init(map, vwd_hashmap_cmp, NULL, 0);
+
+	entry = buf = vwd_data->buf;
+	len = vwd_data->len;
+	for (i = 0; i < len; i++) {
+		if (buf[i] == '\0') {
+			parent_directory_hashmap_add(map, entry, buf + i - entry);
+			entry = buf + i + 1;
+		}
+	}
+}
+
+static int check_directory_hashmap(struct hashmap *map, const char *pathname, int pathlen)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct virtualworkdir vwd;
+
+	/* Check for directory */
+	strbuf_reset(&sb);
+	strbuf_add(&sb, pathname, pathlen);
+	strbuf_addch(&sb, '/');
+	vwd.pattern = sb.buf;
+	vwd.patternlen = sb.len;
+	hashmap_entry_init(&vwd, vwdhash(vwd.pattern, vwd.patternlen));
+	if (hashmap_get(map, &vwd, NULL)) {
+		strbuf_release(&sb);
+		return 0;
+	}
+
+	strbuf_release(&sb);
+	return 1;
+}
+
+/*
+ * Return 1 for exclude, 0 for include and -1 for undecided.
+ */
+int is_excluded_from_virtualworkdir(const char *pathname, int pathlen, int dtype)
+{
+	if (!core_virtualworkdir)
+		return -1;
+
+	if (dtype != DT_REG && dtype != DT_DIR && dtype != DT_LNK)
+		die(_("is_excluded_from_virtualworkdir passed unhandled dtype"));
+
+	if (dtype == DT_REG || dtype == DT_LNK) {
+		int ret = is_included_in_virtualworkdir(pathname, pathlen);
+		if (ret > 0)
+			return 0;
+		if (ret == 0)
+			return 1;
+		return ret;
+	}
+
+	if (dtype == DT_DIR) {
+		int ret = is_included_in_virtualworkdir(pathname, pathlen);
+		if (ret > 0)
+			return 0;
+
+		if (!parent_directory_hashmap.tablesize && virtual_workdir_data.len)
+			initialize_parent_directory_hashmap(&parent_directory_hashmap, &virtual_workdir_data);
+		if (!parent_directory_hashmap.tablesize)
+			return -1;
+
+		return check_directory_hashmap(&parent_directory_hashmap, pathname, pathlen);
+	}
+
+	return -1;
+}
+
+/*
+ * Update the CE_SKIP_WORKTREE bits based on the virtual working directory.
+ */
+void apply_virtualworkdir(struct index_state *istate)
+{
+	char *buf, *entry;
+	int i;
+
+	if (!git_config_get_virtualworkdir())
+		return;
+
+	if (!virtual_workdir_data.len)
+		get_virtual_workdir_data(&virtual_workdir_data);
+
+	/* set CE_SKIP_WORKTREE bit on all entries */
+	for (i = 0; i < istate->cache_nr; i++)
+		istate->cache[i]->ce_flags |= CE_SKIP_WORKTREE;
+
+	/* clear CE_SKIP_WORKTREE bit for everything in the virtual working directory */
+	entry = buf = virtual_workdir_data.buf;
+	for (i = 0; i < virtual_workdir_data.len; i++) {
+		if (buf[i] == '\0') {
+			int pos, len;
+
+			len = buf + i - entry;
+
+			/* look for a directory wild card (ie "dir1/") */
+			if (buf[i - 1] == '/') {
+				if (ignore_case)
+					adjust_dirname_case(istate, entry);
+
+				pos = index_name_pos(istate, entry, len);
+				if (pos < 0) {
+					pos = -pos - 1;
+					while (pos < istate->cache_nr && !fspathncmp(istate->cache[pos]->name, entry, len)) {
+						istate->cache[pos]->ce_flags &= ~CE_SKIP_WORKTREE;
+						pos++;
+					}
+				}
+			} else {
+				if (ignore_case) {
+					struct cache_entry *ce = index_file_exists(istate, entry, len, ignore_case);
+					if (ce)
+						ce->ce_flags &= ~CE_SKIP_WORKTREE;
+				} else {
+					int pos = index_name_pos(istate, entry, len);
+					if (pos >= 0)
+						istate->cache[pos]->ce_flags &= ~CE_SKIP_WORKTREE;
+				}
+			}
+
+			entry += len + 1;
+		}
+	}
+}
+
+/*
+ * Free the virtual working directory data structures.
+ */
+void free_virtualworkdir(void) {
+	hashmap_free(&virtual_workdir_hashmap, 1);
+	hashmap_free(&parent_directory_hashmap, 1);
+	strbuf_release(&virtual_workdir_data);
+}
diff --git a/virtualworkdir.h b/virtualworkdir.h
new file mode 100644
index 0000000000..139d019d44
--- /dev/null
+++ b/virtualworkdir.h
@@ -0,0 +1,25 @@
+#ifndef VIRTUALWORKDIR_H
+#define VIRTUALWORKDIR_H
+
+/*
+ * Update the CE_SKIP_WORKTREE bits based on the virtual working directory.
+ */
+void apply_virtualworkdir(struct index_state *istate);
+
+/*
+ * Return 1 if the requested item is found in the virtual working directory,
+ * 0 for not found and -1 for undecided.
+ */
+int is_included_in_virtualworkdir(const char *pathname, int pathlen);
+
+/*
+ * Return 1 for exclude, 0 for include and -1 for undecided.
+ */
+int is_excluded_from_virtualworkdir(const char *pathname, int pathlen, int dtype);
+
+/*
+ * Free the virtual working directory data structures.
+ */
+void free_virtualworkdir(void);
+
+#endif
